Right Phototactic Orientation

Transcription

1 562 Biophysicl Journl Volume 73 Septemer How Chimydomons Keeps Trck of the Light Once It Right Phototctic Orienttion Hs Reched the Klus Schller, Ruth Dvid, nd Riner Uhl Ateilung Physiklische Biologie der Ludwig Mximilins Universitt, D-8638 Munchen, Germny ABSTRACT By using rel-time ssy tht llows mesurement of the phototctic orienttion of the unicellulr lg Chlmydomons with millisecond time resolution, it cn e shown tht single photons not only induce trnsient direction chnges ut tht fluence rtes s low s photon cell- s- cn lredy led to persistent orienttion. Orienttion is inry vrile, i.e., in prtilly oriented popultion some orgnisms re fully oriented while the rest re still t rndom. Action spectr revel tht the response to pulsed stimulus follows the Drtnll-nomogrm for rhodopsin while the response to persistent stimulus flls off more rpidly towrd the red end of the spectrum. Thus light of 54 nm, for which ch/my-rhodopsin is eqully sensitive s for 44-nm light, induces no mesurle persistent orienttion while 44-nm light does. A model is presented which explins not only this ehvior, ut lso how Chlmydomons cn trck the light direction nd switches etween positive nd negtive phototxis. According to the model the ility to detect the direction of light, to mke the right turn nd to sty oriented, is direct consequence of the helicl pth of the orgnism, the orienttion of its eyespot reltive to the helix-xis, nd the specil shielding properties of eyespot nd cell ody. The model plces prticulr emphsis on the fct tht prolonged swimming into the correct direction not only requires mking correct turn initilly, ut lso voiding further turns once the right direction hs een reched. INTRODUCTION Phototxis mens orienttion prllel to the mient light nd movement towrd (positive phototxis) or wy from (negtive phototxis) the light source (Pfeffer, 94; Buder, 97). To chieve this gol the orgnism hs een ssumed to possess the ility to determine the light direction nd, using some sort of internl signl processing (Foster nd Smyth, 98), to evlute intensity dt nd djust its orienttion ccordingly (Pfeffer, 94; Buder, 97; Feinlei, 975; Foster nd Smyth, 98; Smyth et l., 988; Ruffer nd Nultsch, 99; Witmn, 993). During forwrd-swimming, Chlmydomons uses its two flgell in reststroke-style fshion (Foster nd Smyth, 98; Ruffer nd Nultsch, 985, 99). However, insted of swimming on stright lines, the lg rottes counterclockwise round its longitudinl xis nd thus pursues helicl pth (Buder, 97; Ruffer nd Nultsch, 985; Witmn, 993). In the course of the ody rottion the photoreceptor molecules in the eyespot perceive sinusoidlly modulted light intensity. Modultion decreses when the "right orienttion" is pproched nd vnishes when trcking- nd light-direction coincide (Foster nd Smyth, 98). This hs een ssumed to constitute "trcking signl" tht steers the cells into the right direction; i.e., the cells re thought to determine this mplitude nd to pursue turning ction until the modultion hs vnished. This model, however, does not llow distinguishing etween swimming direction Received for puliction 3 Octoer 996 nd in finl form 28 My 997. Address reprint requests to Riner Uhl, Ateilung Physiklische Biologie der Ludwig Mximilins Universitt, Menzinger Strsse 67, D-8638 Munchen, Germny. Tel.: ; Fx: ; E-mil: uhl@otnik.iologie.uni-muenchen.de C 997 y the Biophysicl Society /97/9/562/ $2. towrd or wy from the light nd does not explin how the switch etween positive nd negtive phototxis is rought out. The following communiction ddresses this prolem nd proposes solution. By using novel ssy tht llows mesurement of phototctic orienttion with high precision nd millisecond time resolution (Schller et l., unpulished oservtions) we hve monitored oth the orienttion process nd the persistence of once-chieved orienttion under conditions where only one or few photons re sored per cell nd second. We found different wvelength dependence for the two processes, similrly to wht hs een noted efore y Nultsch et l. (97), Foster et l. (984), Foster nd Smyth (98), nd ourselves (Uhl nd Hegemnn, 99,), i.e., mesurements tht proe the phototctic migrtion yield n ction spectrum which flls off much more rpidly towrd the red thn mesurements tht proe the ctul orienttion process, i.e., direction chnges. As Foster nd Smyth (98) hve pointed out, using threshold ction spectr cn reduce the difference etween the two diverging results, ut they give no specific explntion for wht could e responsile for this ehvior in the given cse. We hve therefore studied the discrepncy in detil under conditions where oth the orienttion process nd the process of eing nd stying oriented cn e registered simultneously, nd this hs led us to extend existing models of phototxis. Our new model provides n explntion for the ove discrepncy, it explins how orienttion cn e chieved with short flshes of light (Feinlei, 975) or even, s n ll-or-nothing event, y single photons (see elow) nd it offers n explntion for the oservtion tht orienttion my e reched with time course too rpid to mke use of modulted intensity signl: 25 ms fter the onset of orienttion, i.e., fter /2 rottion period, hlf-mximl orienttion is lredy chieved.

2 Schller et l. Phototctic Orienttion in Chlmydomons 563 MATERIALS AND METHODS C. reinhrdtii strin 86 mt- cells were grown on gr pltes for 5 dys nd differentited overnight in nitrogen-free liquid miniml medium (NMM: 8 AM MgSO4, p.m CCl2, 3. mm K2HPO4, 3.4 mm KH2PO4, ph = 6.8 plus trce elements) t cell concentrtion of 7 X 6 cells/ml s previously descried (Hegemnn et l., 988). The phototcticlly most ctive cells were photoselected nd drk-dpted (Uhl nd Hegemnn, 99,) under optiml ertion for t lest nother hour in phosphte uffer contining 3. mm K2HPO4, 3.4 mm KH2PO4,,uM CCl2, nd 8,uM MgSO4 djusted to ph 6.8. Before the experiment, cells were diluted to 6 cells/ml nd plced in cuvette. Fresh liquots were tken for every set of exposures. Strin 86 mt-, which exhiits only negtive phototxis (Hegemnn et l., 988), ws chosen in order to ssure tht only single orienttion response ws otined under ll conditions, thus simplifying the interprettion considerly. The opticl set-up, which hs een descried in detil elsewhere (Schller et l., unpulished oservtions) is schemticlly depicted in Fig. nd in detil in Fig.. Prllel monitoring light of 88 nm ws scttered y the cells in the cuvette nd the scttered light ws collected y I m LED I L L A L BS,4 <-- lens-comintion. In the focl plne of the lens-comintion, 2 detectors (Siemens SFH 23-F) were rrnged on circle, mesuring light scttered t constnt scttering ngle "V' nd on this circle like the ciphers on clock, corresponding to vrile scttering ngle "9". Mximl signls, reflecting the trnsition of the popultion from n unoriented to n oriented stte, were otined t ei = 6 nd n ngle "S" corresponding to the 3 o'clock position. The continuous ctinic light hd ndwidth of 22 nm nd the pulsed ctinic light ws derived from commercil photo-flsh of ms durtion (Metz Mecclitz, Germny). Both ctinic light sources could e coupled into the system y mens of qurtz fier (Fig. ) such tht they either originted from within the detector circle (), fced the detector circle (2 or 3), or cme from one or the other side of it (4 or 5). Photocurrents were converted into voltges using current-to-voltge converters with feedck resistor of MfQ. Their output ws fed into logrithmic mplifiers (Uhl et l., 987) nd susequently smpled y n ITC- 6 (Instrutech) AID-converter, controlled y power Mcintosh computer (Apple) nd the IGOR-Xops, provided y R. Bookmn. Motion nlysis ws crried out under the microscope, using lox ojective, commercilly ville CCD-video cmer, nd the progrm BIOTRACK (Dr. K. Vogel, Germny). Asolute photon exposures were determined using clirted, lrge-re photodiode (S 337-BQ, Hmmtsu). The position of the eyespot reltive to the helicl pth of the orgnism ws determined in n upright microscope, using 63X wter immersion ojective (Zeiss Achropln wter 63x, NA.9). Cells were plced in chmer which hd coverslip s ottom nd ws open on top. The monitoring em (88 nm) ws produced y light-emitting diode (Hitchi HLP 4). It illuminted the cells from elow. The orienting ctinic light ws delivered from one side, using the fier cw-delivery system descried elsewhere (Schller et l., unpulished oservtions), nd the flsh ws pplied from position in the sme plne, i.e., perpendiculr to the opticl xis of the microscope nd 35 wy from the cw-light source. In oth cses the light ws collimted using pproprite lenses. A slow scn CCD-cmer (Thet, Groenzell, Germny) ws plced in the imge plne. Its frme trnsfer structure llows it to cquire two susequent imges spced only.2 ms prt (Messler et l., 996). One imge ws recorded with severl hundred milliseconds exposure time. It ws ment to produce continuous helicl trces of s mny s possile lgl cells. Due to the high mgnifiction, however, only tiny frction of the cells produced clerly visile helicl trce within the nrrow depth of field. A second imge ws tken shortly fter the first one, nd during its exposure flsh of ornge light ws pplied. A frction of the cells tht hd produced helicl trce in one plne lso produced mesurle reflex, nd since the two imges were tken so shortly fter ech other, the position of the reflex could e identified reltive to the helicl pth. To test the effectiveness of ctinic light of vrying wvelength nd ngle of incidence with respect to its ility to induce stop-responses, single cells were sucked into pipette nd held in the specimen plne of the confocl microscope (Schller et l., 997). By repeted sucking nd relese ction the orgnism could e oriented such tht the eyespot directly fced one ojective. This position, which ws considered confirmed when mximl reflexes were seen, [for detils see Schller et l. (997)], served s reference point from which, y rotting the pipette, vrious defined ngles of incidence could e reched. Flshes of monochromtic light were then pplied through the microscope optics nd the frequency of stop-responses ws determined from video recordings. The undultion movement of the two flgell, which sets in -3 ms fter the flsh, could thus e identified esily. FIGURE () Schemtic digrm of the opticl set-up for the determintion of orienttion. () Detiled pln of the opticl set-up. "L" re lenses, "A" n perture stop, "BS" em-splitters, "Cu" the cuvette, "LS" lens comintion, nd "Det" the detector ring. The possile illumintion schemes re numered from to 5. RESULTS AND DISCUSSION Mesuring orienttion of n orgnism exhiiting opticl nisotropy Opticlly homogeneous sphericl ojects exhiit sphericl scttering symmetry (Hecht, 974). A devition from per-

3 564 Biophysicl Jouml Volume 73 Septemer 997 fect sphericl ppernce or nonsymmetricl mss distriution within the oject renders the scttering profile symmetricl. However, popultion of such sctterers produces symmetricl scttering profile when numerous sctterers of rndom orienttion re llowed to contriute to the mesurement. Only in n oriented popultion the symmetricl scttering profile of the individul scttering oject ecomes pprent. As consequence, the trnsition from rndom to n ordered popultion of n inhomogeneous scttering oject cuses trnsition from symmetricl to n symmetricl scttering profile, nd this trnsition cn e used to monitor time course nd extent of the orienttion of n inhomogeneous sctterer. As we hve demonstrted elsewhere, Chlmydomons is such n inhomogeneous sctterer (Schller et l., unpulished oservtions). Fig. 2 shows kinetic trces of the orienttion process in continuous light of 488 nm. When the photon flux density ws incresed from 2 x 5 photons m-2 s-i to 2 x 8 photons m-2 S-, the signls grew igger nd ecme fster. Both slope nd finl mplitude s function of irrdince re est descried y the eqution (Nk nd Rushton, 966). A = AXl /const. + " The exponent n is.3, i.e., reltively close to one. The irrdince required for hlf-mximl response ws 5 X 5 photons m-2 S- when light of 488 nm ws used. Given tht there re -22,-3, rhodopsins per cell (Foster nd Smyth, 98; Uhl nd Hegemnn, 99; Beckmnn nd Hegemnn, 99; Deininger et l., 995), tht their sorption cross-section "E' is close to tht of other rhodopsins, i.e.,.5 X -2 m2 (Foster et l., 984; Smyth et l., 988; Uhl nd Hegemnn, 99) nd tht their photochemicl quntum efficiency is -.66 (Foster et l., 984; Smyth et l., 988), one rhodopsin is photoisomerized per cell nd second under these conditions. A sttisticl exmintion on the sis of electrophysiologicl mesurements of the photoreceptor current (Beck, 996) comes to the sme conclusion, nmely tht t 5 X 5 photons m-2 s the cells re deling with individul photon sorption time (s) FIGURE 2 Time course of the orienttion process mesured t different irrdinces (3 o'clock detector, wvelength of the ctinic light: 5 nm). The irrdinces were, from top to ottom:.7 X i4, 6.3 X 4, X i'5,.7 x io'5, 3.2 x 5, 6.3 X 5, X 6,.7 X 6, 3.2 x 6, 6.3 X 6 photons m-2 s'. events. The nerly liner increse in signl mplitude with incresing irrdince must therefore rise from n incresed numer of cells tht hve "seen" single photon nd tht the sorption of two photons does not led to significntly etter orienttion. This conclusion, which my not e ovious t first sight, is direct consequence of the quntl nture of light: since the sorption of single photons is n ll-or-nothing event, popultion response, whose extent is grded with photon exposure under conditions where, on verge, only single photons re sored, must reflect n incresed frction of cells contriuting to the popultion response (Uhl nd Hegemnn, 99). To test the ssumption tht two sored photons do not produce much etter orienttion thn one, we hve exmined the orienttion process of single cells under the microscope. Fig. 3 shows microscopic trces of cells in rndomly oriented popultion nd sttisticl nlysis of their orienttion, wheres Fig. 3 shows the sme for oriented cells. The rodness of the ngulr distriution of swimming directions in drkness nd t three different irrdinces is shown in Fig. 3 c. The etter overll orienttion t higher irrdinces does not men tht the individul cells were oriented etter, it merely mens tht lrger frction of the cells ws oriented. All oriented cells, however, ppered to hve the sme degree of orienttion (Fig. 3 d). This lends further support to the notion tht the orienttion process is inry sttisticl event, which cn e evoked y the sorption of single photons. Phototctic ction spectrum in continuous light The identifiction of Chlmy-rhodopsin s the sole photoreceptor in Chlmydomons rests mostly on ction spectroscopy (Foster nd Smyth, 98; Uhl nd Hegemnn, 99; Hrz nd Hegemnn, 99; Kroger nd Hegemnn, 994). However, while it ppers to e proven eyond dout tht the photoreceptor of the stop response is rhodopsin with n sornce mximum -49 nm (Hrz nd Hegemnn et l., 99; Zcks et l., 993), there is still no definite proof tht the photoreceptor responsile for phototxis is the sme. We hve therefore tried to mesure n ction spectrum for the degree of orienttion chieved using very dim stimuli of continuous light. Fig. 4 depicts stimulus response curves for the degree of orienttion s determined y light scttering experiments. In Fig. 4 ctinic light of 488 nm nd elow ws used, wheres in Fig. 4 ctinic light of 488 nm nd ove ws pplied. The light titrtions in Fig. 4 yielded the expected results, i.e., the curves were prllelshifted nd their sensitivity ws highest t 488 nm nd decresed with decresing wvelength. From these dt one could construct the low-wvelength til of n ction spectrum in complete greement with the Drtnll-nomogrm for rhodopsin (Drtnll, 972). The stimulus response curves in Fig. 4, however, cquired t wvelengths >49 nm, exhiited totlly different ehvior. Not only were the curves shifted, they ecme compressed the more the ctinic

4 Schller et l. Phototctic Orienttion in Chlmydomons 565 c drk -i",.2 o. CD. o CD ) m).3 _ photons/ M2s PM -F c.2 - c;. cix.c ) FIGURE 3 () Swimming trces of unoriented cells nd their ngulr orienttion profile. The ltter ws determined from the numer of prtil trces (durtion of 5 ms) tht hd prticulr direction. The directionl in width ws ± 3.8. () Swimming trces of oriented cells nd their ngulr orienttion profile. Bin width ± 2. (c) Angulr profile of the frctionl degree of orienttion of single cells t four different irrdinces. The "frctionl degree of orienttion is defined s the frction of cells eing oriented t given ngle ± 2. The solid line is Gussin fit. (d) Superimposed ngulr profile (Gussin fit) of the frctionl orienttion, showing tht the hlf-width is reltively independent of the irrdince. d k.. 6uwk.&-& 2*7 photons/ m2s ni swimming direction () c c) L- c) o Cs ) 4).o swimming direction () wvelength ws shifted towrd the yellow prt of the spectrum. Light of 53 nm, which should hve the sme efficiency to excite rhodopsin s 45-nm light, hd gretly reduced efficiency with respect to the orienttion process, nd 55 nm yielded hrdly ny orienttion t ll. This cnnot e ccounted for y photochromic effects, since under conditions where only single photons were sored per cell nd second, photolysed rhodopsin will, on verge, hve to wit five hours efore it "sees" nother photon. Prllel experiments performed on single cells under the

5 566 Biophysicl Jouml o m -.5 ni -. photon flux density [ m-; s I uj _ _ photon flux density [ m - s ] FIGURE 4 Light-titrtion of the degree of orienttion induced y continuous light of wvelengths etween () 4 nd 488 nm nd etween () 488 nd 55 nm. The rrow mrks the photon flux t which photon is sored per cell nd second t 488 nm, i.e., 5 X 5 photons m-2 s-. microscope showed tht there ws still some orienttion in yellow light, ut tht the ngulr distriution ecme progressively roder with incresing wvelength (dt not shown). The ove dt mke it very difficult to construct meningful ction spectrum ove 52 nm. Even though ction spectr should lwys e constructed from threshold experiments, fct tht hs een stressed repetedly y Foster nd collegues [Foster et l. (984); Smyth et l. (988)], one wonders how threshold dt hve een otined ove 53 nm y these uthors given the low degree of orienttion ttined with yellow light, no mtter how dim. Our results re reminiscent of the ction spectrum of Nultsch et l. (97), which exhiits shrp decline in sensitivity ove 5 nm nd hrdly ny sensitivity ove 54 nm. It ws derived from migrtion distnce dt. Clerly, when there is no persistent orienttion, s indicted y the present study, there cnnot e net movement in one direction. Phototctic ction spectrum under pulsed light conditions The first step in phototxis is light-induced direction chnge (Pfeffer, 94; Buder, 97). In previous study Volume 73 Septemer 997 sed on light-scttering trnsients otined from n unoriented popultion, we hve proposed tht direction chnges cn e induced y single photons (Uhl nd Hegemnn, 99,). At tht time we did not understnd the physicl ckground of the signls, which we now do. Moreover, y preorienting the popultion, much lrger signls cn now e otined. They reflect light-induced disturnces of n existing orienttion. A dim flsh of green light (2 X 6 photons m-2) ws pplied perpendiculrly to the orienting light. It led to trnsient disturnce of the equilirium orienttion which mnifested itself s trnsient light-scttering intensity chnge. The periodicity seen in the flshinduced trces hd frequency of -2 Hz, very close to the rottionl frequency of the helicl movement (Foster nd Smyth, 98; Ruffer nd Nultsch, 985). This "tumlingresponse" cn lso e evoked y the sorption of single photons. We hve determined n ction spectrum for such single photon-induced disturnces using pulsed light source nd n "equl response" pproch. To mke sure tht the wvelength dependence of the shding due to eyespot pprtus nd the other cell pigments did not pertur the mesurement; the mesurement ws performed under conditions where the photoreceptors ssumed constnt ngle with respect to the ctinic light, i.e., cells were oriented such tht they swm directly towrd the flsh. With these precutions tken, dim flshes, i.e., photon exposures tht produced only one or few photoisomeriztions per cell, produced smll chrcteristic signls which exhiited n identicl shpe t ll wvelengths nd differed only in their sensitivity (the reciprocl of the exposure required for given response reflects the sensitivity of the system t given wvelength). An ction spectrum constructed this wy (Fig. 5) exhiits the expected rhodopsin shpe nd fits the Drtnll nomogrm. So from these dt there is no evidence for C7,)E C.. C.._ um ~~~~~ M.e -W qcn. i wvelength [nm] FIGURE 5 Action spectrum of flsh-induced trnsient orienttion chnges compred with the Drtnll nomogrm. Sensitivity ws defined s the inverse of the exposure required for given, ove-threshold trnsient response. The five symols correspond to five different experiments tht were crried out on different dys. Cells were oriented y green light (5 X nm, 6 photons m-2 s-) which fced the detector ring nd the perturing flsh ws pplied from the opposite direction. Il

6 Schller et l. the involvement of ny other photoreceptor in the onset of phototctic orienttion. The discrepncy with respect to the orienttion in continuous light, however, remins to e elucidted nd we feel tht it is key issue for the understnding of phototxis. Phototctic Orienttion in Chimydomons 567 On the geometry of the helicl pth nd its significnce for oriented swimming To id the following discussion we wnt to recpitulte some sic fetures of the helicl swimming of Chlmydomons: Chlmydomons uses its two flgell in reststroke-like fshion (Ruffer nd Nultsch, 985). A different et efficiency of the two flgell (Ruffer nd Nultsch, 985) would not explin why the orgnism swims on helicl pth, it would only mke it swim in circles. There is, however, n dditionl rottion round the longitudinl cell xis, which turns the circulr movement into helicl one. While the resons for this cell rottion re not yet cler (Ruffer nd Nultsch, 985, propose n out-of-plne component of the et), it is known tht this rottion is counterclockwise, nd determines the hndedness of the resulting helix. Another logicl consequence of the symmetricl eting pttern leding to helicl movement is tht the two flgell re lwys rdilly oriented with the dominting flgellum pointing to the outside. Single sored photons cn cuse reorienttion of cell popultion. Since for n oriented cell ny further turn is detrimentl, it needs to keep its photoreceptors protected from ny further photon sorption once it is oriented. Consequently, the oriented stte must e stte of miniml photon sorption proility if it is to constitute stle equilirium stte. Imgine-in nlogy to n energy hypersurfce in physics- "photonsorption hypersurfce" which descries the proility of photon cpture s function of the two ngulr orienttions e nd cp reltive to the direction of light (Fig. 6 ). This photonsorption hypersurfce must ssume minimum when trcking direction nd light direction coincide. If this stte hs een reched, the signl tht tells the orgnism not to turn nymore is not the now-reched constncy of perceived light intensity, which needs to e interpreted y the orgnism, ut it is the sence of n internl, light-induced signl. As we hve stted previously, oth flgell re lwys rdilly oriented with respect to the helicl xis. This mens tht while they ssume constnt ngle e reltive to the helix xis, their ngle p precedes round it, ssuming ngles etween nd 36. This holds for ny point on the surfce of the orgnism, including the eyespot locted close to the equtor of the cell. If it were locted within the eting plne of the two flgell, the "viewing direction" of the eyespot nd the light would form right ngle for cells swimming towrd or wy from the light. So while under these conditions the modultion of the perceived intensity would vnish for n oriented cell, it would neither e miniml, nor would it llow the orgnism to determine trns-flgellum dominnce of: cis-flgellum FIGURE 6 () Definition of the two ngles e nd sp which descrie the viewing direction (directivity) of the eyespot reltive to the direction of light. () Viewing the helicl swimming pth of trns- respectively cis-dominted lg, moving directly towrd the oserver. A drk shded eyespot mens tht it cn e directly viewed y the oserver, while lightly shded eyespot mens tht there is the cell ody etween eyespot nd oserver. whether it ws swimming towrd or wy from the light. The proility of photon cpture would only e ttenuted y fctor of two for unpolrized light compred to the fully exposed orienttion, nd consequently the chnce of further direction chnges would e high. Following this line of resoning it now mkes sense why evolution hs plced the eyespot -45 outside of the eting plne. A direct consequence of this "geometricl trick" is tht cell swimming prllel to the light direction, i.e., for which trckingnd light-direction coincide, still perceives constnt light intensity over helicl period. However, four possile configurtions need to e distinguished now s demonstrted in

7 568 Biophysicl Journl Volume 73 Septemer 997 Fig. 6 : the cell my swim towrd or wy from the oserver, nd either the cis- or trns-flgellum my e dominting. In ll four cses the orienttion of the eyespot reltive to the helicl xis is invrint, ut due to the 45 etween eyespot nd eting plne two possile orienttions of the eyespot reltive to the swimming direction exist, depending on whether the cis- or the trns-flgellum ets stronger. If trns domintes, light originting from the position of the oserver cn lwys rech the photoreceptors directly, while light coming from the opposite site hs to pss through prt of the cell nd the interference reflector efore it strikes the photoreceptor molecules. On this pssge the light intensity gets ttenuted to vrile degree, s we know from microspectrophotometric mesurements (Schller et l., 997). The "directivity" of the eyespot therefore ttins forwrd or ckwrd component, exposing, respectively protecting it from the light. Since only shielded photoreceptors gurntee tht the right orienttion is stle equilirium position, this stte of miniml, constnt proility of photon cpture must e ssumed when the cells re oriented. So negtively phototctic lge must swim with forwrd rked eyespot, i.e., with dominnt trns-flgellum. This immeditely explins the poor orienttion chieved with yellow light: even though the eyespot is rked forwrd, i.e., wy from the light, the poor shielding provided y cell ody nd eyespot (Schller et l., 997) mkes the proility of photon cpture only slightly reduced when the cell is oriented. In fct, we cn even rtionlize the oservtion tht the slight orienttion chieved with.5 X 6 photons m-2 s- disppers gin t higher photon flux densities, If, due to poor shielding, the stimulus/response curve for orienting direction chnges in orgnisms with mximlly exposed photoreceptors is only slightly shifted to the left compred to the stimulus/response curve for disorienting direction chnges in orgnisms which swim wy from the light, the chnce to mintin the right orienttion is the difference etween the chnces for orienttion nd disorienttion. It is ound to ssume mximum etween the two hlf-points of the respective stimulus/ response curves. This is the cse s cn e judged from the ction spectrum mesured under pulsed light conditions where t 55 nm hlf-mximl signl mplitudes require.5 X 6 photons m-2 s-. Moreover, s the height of this mximum should get smller the closer the two curves pproch ech other, i.e., the lower the contrst, the results in Fig. 4 would indicte tht t 55 nm the contrst rtio must e <.5. This prediction is lso met, s will e shown elow. Mesuring the degree of protection for popultion of oriented cells To test the vlidity of the ove prepositions we first wnted to demonstrte directly tht oriented cells protect their photoreceptors from the orienting. A popultion of strin 86, which, under ordinry light conditions shows exclusively negtive phototxis (Uhl nd Hegemnn, 99), ws oriented using lue continuous light. A "disturing flsh" of vrious wvelengths ws then pplied from three different directions (Fig. 7 ) nd the threshold sensitivity, i.e., the exposure t which mesurle turning ction ecme mnifest, ws determined. The resulting sensitivity curves (Fig. 7 ) were normlized with respect to the ction spectrum (Fig. 6 ). Light originting from the direction into which the lge were swimming ws found to e much more effective thn light coming from the opposite direction. The difference ws mximl in the lue nd declined towrd the red. Light coming from the side, on the other hnd, ws similrly effective s light coming from the front direction, with distinct reltive sensitivity mximum round 55 nm, the pek of reflectivity of the interference reflection. The fct tht the "contrst rtio," i.e., the sensitivity for hed-on nd for ck-illumintion, is only 4 in the lue, i.e., lower thn wht the microspectrophotometric sornce mesurements hve indicted (Schller et l., 997), is not surprising: not ll cells of the popultion re perfectly oriented, nd the "unoriented" frction of the popultion is not s well protected from hed-on photons s -_ tf Cl). c front flsh side flsh "front-flsh" - - "ck-flsh" -h--- 'side-flsh" Detectorsl ck flsh -- x-}.' - ' wvelength [nm] FIGURE 7 () Three excittion configurtions for n ctinic flsh m disturing the orienttion of n oriented popultion. Cells were preoriented with 5 nm light ( X 6 photons m-2 s-') nd flshed with wvelengths etween 425 nd 575 nm from the front (), from the side (), nd from the ck (c). [] denotes the cuvette nd l-... the detector-rry. () Wvelength-dependent sensitivity of the three configurtions, normlized to the rhodopsin spectrum (tken from Fig. 4).

8 Schller et l. the oriented frction. Consequently there will e response from these "nughty" cells t lower exposures thn from their oriented counterprts, nd this could led to n overestimtion of the sensitivity. An dditionl explntion could e tht the sornce of the cell is less when the light reching the photoreceptor lyer does not hve to cross the cell rdilly ut t very shllow ngle, s is the cse in Chlmydomons. Phototctic Orienttion in Chlmydomons Geometricl requirements for optiml protection of photoreceptors in oriented cells Given the geometry of the lg nd its helicl pth, successful shielding ction is only gurnteed for n orgnism swimming wy from the light when the trns-flgellum domintes nd the eyespot is rked in forwrd/inwrd direction. This ws experimentlly verified. By using infrred light the helicl movement of single lge could e monitored under the microscope with slow-scn CCD cmer, which ws llowed to integrte for severl seconds. Provided the orgnism ws stying within the focus plne of the ojective- rrely encountered sitution-the helix could e visulized s sinusoidl projection. Severl thousnd of such trjectories were recorded from oriented strin 86 cells, swimming wy from continuous lue light. The experiment ws terminted y flsh of yellow light (reflectivity of the eyespot is est t 55 nm, see Schller et l., 997), which ws synchronized with the recording of second frme. The resulting imge ws serched for reflexes of the flsh, which could only e seen when the eyespot ssumed n ngle hlfwy etween tht of the incoming light on one hnd nd the cmer on the other hnd (Fig. 8 ). Thus it ws possile to determine t which position reltive to the helicl period fvorle, i.e., reflecting orienttion, of the eyespot ws reched. The result shown in Fig. 8 supports the ove proposition tht the eyespot is rked forwrd/inwrd nd tht the trns-flgellum domintes in n orgnism swimming wy from the light. To determine the exct degree of shielding for the orienttion of the eyespot where shielding is most needed (i.e., when the cell is oriented) nd for ll other orienttions, we hve crried out "psychophysicl test" on single cells, so-clled "frequency of seeing" experiment. Frequency of seeing experiments were first crried out y Hecht et l. (942) in order to determine the photon requirement of humn vision. They pplied stimuli contining known numer of verge photons nd determined the proility with which the stimulus ws perceived y test person. By incresing the stimulus rightness, stimulus-response curves were recorded nd their shpe ws used to determine how mny photons were required for stimulus to ecome noticele. We hve crried out similr experiments with lgl cells held with micropipette. The cells were rotted with respect to the incoming light in order to determine how the photon requirement chnged with orienttion reltive to the stimulting light. Since our test specimen could not e interrogted directly whether it hd seen light, we used one 2'.8 x -.6 CD U DIMN. V VA I. M. >'-.2-2 V7AIVYAIMZ 36 r, 2! helicl phse ngle FIGURE 8 () Geometry of the experiment for testing the orienttion of the eyespot reltive to the helicl pth. The orienting light hd wvelength of 48 nm ( x 6 photons m-2 s-') nd the flsh of 55 nm ( x 9 photons m-2). () Success rte for the detection of flsh reflex from the eyespot s function of the helicl phse ngle. of its photoresponses, i.e., the stop response, which could e identified visully. The stop response is photophoic rection exhiited y Chlmydomons t elevted light levels (Schmidt nd Eckert, 976; Hegemnn nd Bruck, 989). It occurs in n ll-or-nothing fshion when the primry cell depolriztion exceeds criticl level (Hrz nd Hegemnn, 99; Hrz et l., 992), nd it consists of n undultion movement of the flgell, which is esily detected under the microscope. Light titrtions of this sttisticl event (Fig. 9 ) showed n orienttion-dependent sensitivity. Mximum protection ws chieved (Fig. 9 ) when eyespot nd light-direction formed n ngle similr to the one encountered during oriented swimming, lending further support for the vlidity of the model. In greement with microspectrophotometric mesurements (Schller et l., 997) the contrst-rtio ws mximl (8-) in the lue/ green nd very low in the yellow prt of the spectrum. An extended model for phototxis We hve shown ove tht negtively phototctic lg ssumes stte of constnt, miniml photon sorption

9 57 Biophysicl JouJrnl C W - C. co 4-( C C. 6 o U5, U, : en C U I photons m2 I ngle of rottion I4 -I ; l' \%. ii J ngle of rottion FIGURE 9 () Light-titrtions of the frequency of stop-responses t two different wvelengths (54 nd 54 nm). Solid lines men tht the ctinic light hit the eyespot directly () while dotted lines men tht the light hd to pss through the cell first efore reching the eyespot (8). () Proility of flsh perception (light-sensitivity for stop-response) s function of the ngulr orienttion of the eyespot. Aove 8 only few control dt points were cquired in order to verify tht the sensitivity curve is mirror imge of the dt tken elow 8. Given the deliccy of the mesurement the reproduciility of the 36 vlue compred to the vlue is remrkle. The wvelength of the ctinic light ws 54 nm. (c) Proility of flsh perception (light sensitivity for stop-response) s function of the ngulr orienttion of the eyespot t three different wvelengths (464 nm, 54 nm nd 54 nm). In order not to oscure the dt only verge curves re displyed. The vrince ws comprle to the one shown in Fig. 9. Volume 73 Septemer 997 proility when it is oriented prllel to the light nd when it is swimming wy from the light source. In order to do so there is only one single requirement to e met: the trnsflgellum must et stronger ecuse only this wrrnts to n inwrd/forwrd orienttion of the eyespot. Clerly for nturlly positively phototctic orgnism it must e the other wy round, i.e., the eyespot must point to the outside of the helix nd must e tilted ckwrd. This ws not shown experimentlly, ut from the ove it should e ovious tht the orgnism hs no other chnce to keep the eyespot in shielded position. Generlly, in order to mintin constnt orienttion nd to void further direction chnges, in positive phototxis the cis-flgellum, nd in negtive phototxis the trns-flgellum, must dominte. Thus we predict tht the discrepncy found in the literture regrding the reltive position of the eyespot (for discussion see Kreimer, 994) cn e resolved y ssuming tht the uthors plcing the eyespot on the outer side of the helix (Kmiy nd Witmn, 984) were oserving positive phototxis while the others (Diehn, 979; Ruffer nd Nultsch, 987) were viewing negtive phototxis. The ove model my e extended (Fig. ) so s to explin the switch from positive to negtive phototxis exhiited y wild-type cells. Besides the principles outlined ove we only need the finding of Kmiy nd Witmn (984) tht the internl clcium concentrtion regultes the eting pttern of the two flgell in differentil wy. In drkness the resting clcium concentrtion is lowest nd the cis-flgellum ets stronger. The eyespot is hence outwrd/ ckwrd oriented during this time, which we would like to term "phse I." Asorption of few photons leds to modertely elevted clcium level nd concomitnt dominnce of the trns-flgellum (phse II). This cuses the cell to initite turn towrd the light, in the course of which the rhodopsins re trnsiently protected ginst the light with concomitnt drop in internl clcium concentrtion. This my e viewed s the step-down, which is prt of the Ruffer C:._ zc)._ o log [Cji ] FIGURE Hypotheticl model for the switch from positive to negtive phototxis. Shown re hypotheticl clcium inding isotherms for the two flgell nd clcium-concentrtion domins in which the cis- respectively trns-flgellum is dominting.

10 Schller et l. Phototctic Orienttion in Chlmydomons 57 nd Nultsch model (99). In ny cse, it leds to resumption of the cis-flgellum dominnce nd return to phse I. As we hve pointed out ove, cis-dominnce mens tht the eyespot fces ckwrd nd is thus protected, hence pursuing pth towrd the light keeps the eyespot protected nd the clcium concentrtion remins low. At elevted light levels the cells proceed directly from phse I to phse III, i.e., the internl clcium concentrtion rises eyond the level where the trns-flgellum domintes up to level where the cis-flgellum domintes gin. This is no specultion, it merely descries the oservtion tht for negtive phototxis turn wy from the light is required nd tht invrily necessittes trnsient dominnce of the cis-flgellum. We now propose tht higher internl clcium concentrtions re required for the cisflgellum to regin its dominnce, nd these re only reched when the photoreceptors in the eyespot re exposed to the light. As soon s the photoreceptors ecome protected gin, however, which occurs when cell ody nd eyespot shde the photoreceptors, [Ci] returns to lower level where the trns-flgellum domintes gin (phse II). This then constitutes new stle equilirium stte only when the cells pursue swimming pth wy from the light. Before this stte of stle orienttion is reched the cell my crry out one or severl turns, i.e., the model predicts the existence of short tril nd error phse in which the cells switch etween swimming towrd the light or wy from it; the shorter this period is, the etter the contrst. We hve previously descried such oscilltions etween forwrd nd ckwrd swimming (Uhl nd Hegemnn, 99, Fig. 4) nd we hve noticed tht the tril nd error phse is negligile when the cells re exposed to lue light (good contrst), ut cn lst for severl seconds in yellow light. Using the ove model, how cn we envisge (under norml conditions) purely negtively phototctic orgnism like strin 86? The esiest explntion would e tht the resting clcium concentrtion in strin 86 is lredy so high tht the trns-flgellum domintes in the drk. A further increse in clcium cn cuse only one rection, trnsient dominnce of the cis-flgellum, which cuses turn wy from the light, nd return to "trns-dominting conditions" in the drk or when the photoreceptors re protected. Alterntively one hs to ssume tht the clcium sensitivity of the two flgell is incresed in strin 86 such tht t resting clcium level II is lredy reched. Currently we cnnot distinguish etween the two possiilities. In n erlier study we hve descried light dpttion in Chlmydomons. (Uhl nd Hegemnn, 99). When strin 86 cells were light-dpted for extended periods of time, using right white light, they susequently showed trnsient positive phototctic ehvior which decyed with time course of - min. Within the frmework of our model this implies tht during light dpttion the mechnism y which the cell pumps out clcium ws ccelerted. Thus, when the dpting light ws turned off, [Cj] presumly dropped elow its norml resting level, now reching level I, where the cis-flgell dominted. This is the requisite for positive phototxis. The decy of the positive phototxis then implies tht the internl clcium concentrtion grdully pproched its norml resting level gin. According to the ove model phototctic orienttion is governed y the orienttion-dependent contrst rtio of the eyespot pprtus. In the specimen exmined here contrst ppers to rise predominntly due to sornce nd only to minor degree due to reflexion. Moreover, the sornce due to chlorophylls in the cell ody ppers to ply greter role thn the sornce due to crotenoids in the eyespot pprtus. This hs een concluded efore from the fct tht eyespot-less mutnts cn exhiit phototctic ehvior (Morel-Lurens nd Feinlei, 983; Sineshchekov et l., 989). It should e noted, however, tht while contrst rtio of 8-, s found for Chlmydomons strin 86 t 5 nm nd elow, leds to optiml orienttion, much smller contrst of.5-2-strin 86 yields this contrst in yellow light (Schller nd Uhl, 997)-still leds to some residul orienttion which is sufficient for net directed movement. This explins why even chlorophyll-less mutnts cn still move phototcticlly (Kreimer et l., 992; Kreimer, 994). An solutely necessry requirement for the model is constnt position of the eyespot pprtus reltive to the eting plne of the flgell. According to Kreimer (994), who hs studied gret vriety of flgellte lge, this ppers to e the cse, i.e., the eyespot pprtus is lwys outside the flgellr eting plne. There re reports, however, tht the eyespot is not lwys locted perfectly on the cell equtor (Kreimer, 994). According to our model precise loction on the equtor is optiml for n orgnism tht exhiits oth positive nd negtive phototxis. A disloction towrd the posterior prt of the cell increses the shding for light coming from up-front in n oriented positive phototctic orgnism, wheres disloction towrd the nterior prt would e counterproductive for positive, ut helpful for negtive phototxis. The fct tht in strin 86 the eyespot is in most cses in the nterior prt, seldom t the equtor nd even more seldom in the posterior prt (Kreimer, personl communiction) is tken s further evidence for the vlidity of the model. Our more physiclly oriented lortory is not prepred to test the hypothesis on gret vriety of orgnisms; however, we feel confident tht the principles outlined ove will turn out to ply n importnt role in the mechnism of phototctic orienttion, not only in Chlmydomons. P. Hegemnn's help in erlier stges of the work re grtefully cknowledged. The uthors lso thnk H. Hrz nd G. Kreimer for criticl reding of the mnuscript nd numerous vlule suggestions. This work ws supported y grnt of the Deutsche Forschungsgemeinschft. REFERENCES Beck, C Loklistion und Eigenschften lichtinduzierter Ionnstrome in Chlmydomons. Ph.D. Thesis, Ludwig Mximilins Universitt, Munchen.

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