Photoactivated adenylyl cyclase controls phototaxis in the flagellate Euglena gracilis

Distinct Responses of Chloroplasts to Blue and Green Laser Microbeam Irradiations in the Centric Diatom Pleurosira laevis

The centric diatom Pleurosira laevis is a large unicellular alga, in which ca 200 chloroplasts migrate toward the nuclear cytoplasm through the transvacuolar cytoplasmic strands in response to blue-light irradiation and, on the contrary, toward the cortical cytoplasm in response to green-light irradiation. We analyzed these light-induced chloroplast migrations using a scanning laser microbeam provided by a confocal microscope for intracellular irradiation. Spot irradiation of a blue laser microbeam induced rapid assemblage of chroloplasts into the nuclear cytoplasm regardless of the spot position and spot number. On the other hand, one or two spots of green laser microbeam induced chloroplast accumulation at the spots, although increasing spot numbers suppressed chloroplast accumulation at each spot. In our experimental condition, ca 1 min of blue-light irradiation was sufficient to stimulate movement, whereas green-light irradiation required uninterrupted and longer irradiation time (ca 15 min). Chloroplast assemblage induced by blue-light required extracellular Ca2+, and was inhibited by Ca2+ channel antagonists. Furthermore, higher efficiencies of chloroplast migration were obtained when a single beam spot was fragmented and scattered over wider area of plasma membrane. These observations suggested that blue-light induced a response at the plasma membrane, which subsequently activated Ca2+ permeable channels. This sequence of physiological events is identical to what was previously observed with chloroplast movement in response to mechanical stimulation. Furthermore, experiments with the cytoskeleton-disrupting agents, colchicine and cytochalasin D, indicated that blue-light-induced chloroplast movement required microtubules whereas the green-light-induced response to beam spot required actin filaments.

PAR and UV Effects on Vertical Migration and Photosynthesis in Euglena gracilis†

Recently it was shown that the unicellular flagellate Euglena gracilis changes the sign of gravitaxis from negative to positive upon excessive radiation. This sign change persists in a cell culture for hours even if subsequently transferred to dim light. To test the ecological relevance of this behavior, a vertical column experiment was performed (max. depth 65 cm) to test distribution, photosynthetic efficiency and motility in different horizons of the column (surface, 20, 40 and 65 cm). One column was covered with a UV cut-off filter, which transmits photosynthetically active radiation (PAR) only, the other with a filter which transmits PAR and UV. The columns were irradiated with a solar simulator (PAR 162 W m(-2), UV-A 32.6 W m(-2), UV-B 1.9 W m(-2)). The experiment was conducted for 10 days, normally with a light/dim light cycle of 12 h:12 h, but in some cases the light regime was changed (dim light instead of full radiation). Under irradiation the largest fraction of cells was found at the bottom of the column. The cell density decreased toward the surface. Photosynthetic efficiency, determined with a pulse amplitude modulated fluorometer, was negligible at the surface and increased toward the bottom. While the cell suspension showed a positive gravitaxis at the bottom, the cells in the 40 cm horizon were bimodally oriented (about the same percentage of cells swimming upward and downward, respectively). At 20 cm and at the surface the cells showed negative gravitaxis. Positive gravitaxis was more pronounced in the UV + PAR samples. At the surface and in the 20 and 40 cm horizons photosynthetic efficiency was better in the PAR-only samples than in the PAR + UV samples. At the bottom photosynthetic efficiency was similar in both light treatments. The data suggest that high light reverses gravitaxis of the cells, so that they move downward in the water column. At the bottom the light intensity is lower (attenuation of the water column and self shading of the cells) and the cells recover. After recovery the cells swim upward again until the negative gravitaxis is reversed again.

Heterologous expression of photoactivated adenylyl cyclase (PAC) genes from the flagellate Euglena gracilis in insect cells

The unicellular, green flagellate wild-type Euglena gracilis (strain Z) possesses two genes of the photoactivated adenylyl cyclase (PAC) family. The corresponding gene products were found to be responsible for step-up (but not step-down) photophobic responses as well as both positive and negative phototaxis. The proteins consist of two PACalpha (Mr 105 kDa) and two PACbeta (90 kDa) subunits. In an effort to produce sufficient amounts of PAC proteins, several routes of over-expression have been tried including homologous expression in Euglena and heterologous expression in Escherichia coli. All these approaches were hampered by low yield or formation of inclusion bodies. Therefore we decided to attempt a heterologous expression in an insect cell line. PACalpha and PACbeta were separately cloned in the transfer vector pBacPAK9 with a His tag attached. The transfer vector was subsequently cotransfected via baculovirus into the insect cells and amplified. For the expression both recombinant viruses (containing PACbeta and PACbeta, respectively) were cotransfected simultaneously into insect cells. The expressed proteins were analyzed in Western blots using PACalpha and PACbeta antibodies. Most of the proteins were found to be in soluble form in high yield. The recombinant PAC proteins were purified via their attached His tag on an anti-His resin. Adenylyl cyclase activity was quantified after blue-light excitation using a cAMP enzyme immunoassay kit.

Phytochrome-like responses in Euglena: A low fluence response that reorganizes the spectral dependence of the high irradiance response in long-day photoperiodic induction of cell division

Irradiance spectra change spatiotemporally, and angiosperms adapt accordingly, mainly through phytochromes. This study challenges the long-held belief that the flagellated alga Euglena gracilis lacks phytochromes and is therefore unaffected by spectral changes. We photoautotrophically cultured the alga under continuous light (LL), then transferred it to darkness. After about 26h in darkness, different irradiations for 3h enabled cell division in dark-arrested G2 cells evoking a high-irradiance response (HIR). The spectral characteristics of the irradiation during the LL period (pre-irradiation) defined the spectral sensitivity in the subsequent dark period. LL with light rich in the red spectrum led to a HIR to the red spectrum (R-HIR), whereas light rich in the far-red spectrum (FR) led to a FR-HIR. Finishing the period of pre-irradiation consisting of continuous cool-white fluorescent light (rich in R) by a FR pulse enhanced the characteristics of the FR-HIR 26h later. By contrast, a R pulse given at the end of the pre-irradiation rich in FR potentiated the R-HIR. The effects were completely photoreversible between R and FR with critical fluences of about 2mmolm(-2), satisfying the classic diagnostic feature of phytochromes. The action spectrum of the FR effect at the end of pre-irradiation consisting of continuous cool-white fluorescent light (rich in R) had a main peak at 740nm and a minor peak at 380nm, whereas antagonization of the FR effect had a main peak at 640nm and a minor peak at 480nm. Wavelengths of 610 and 670nm appeared in both spectra. We also demonstrated the photoreversibility of 380/640, 480/740, and (610 and 670)/(640 and 740) nm. We conclude that Euglena displays phytochrome-like responses similar to the 'shade avoidance' and 'end-of-day FR' effects reported in angiosperms.

Indications for acceleration-dependent changes of membrane potential in the flagellate Euglena gracilis

The effects of the calcium sequester EGTA on gravitactic orientation and membrane potential changes in the unicellular flagellate Euglena gracilis were investigated during a recent parabolic-flight experiment aboard of an Airbus A300. In the course of a flight parabola, an acceleration profile is achieved which yields subsequently about 20 s of hypergravity (1.8 g(n)), about 20 s of microgravity, and another 20 s of hypergravity phases. The movement behavior of the cells was investigated with real-time, computer-based image analysis. Membrane potential changes were detected with a newly developed photometer which measures absorption changes of the membrane potential-sensitive probe oxonol VI. To test whether the data obtained by the oxonol device were reliable, the signal of non-oxonol-labelled cells was recorded. In these samples, no absorption shift was detected. Changes of the oxonol VI signals indicate that the cells depolarize during acceleration (very obvious in the step from microgravity to hypergravity) and slightly hyperpolarize in microgravity, which can possibly be explained with the action of Ca-ATPases. These signals (mainly the depolarization) were significantly suppressed in the presence of EGTA (5 mM). Gravitaxis in parallel was also inhibited after addition of EGTA. Initially, negative gravitaxis was inverted into a positive one. Later, gravitaxis was almost undetectable.

High irradiance responses involving photoreversible multiple photoreceptors as related to photoperiodic induction of cell division in Euglena

Little is known about the photoreceptors involved in the photoperiodism of unicellular organisms, which we elucidated by deriving their action spectra. The flagellated alga Euglena gracilis exhibits photoperiodism, with a long-day response in cell reproduction. The underlying clock is a circadian rhythm with photoinductive capability, peaking at subjective dusk and occurring at the 26th hour in continuous darkness (DD) when transferred from continuous light (LL); it regulates photoinduction, a high-irradiance response (HIR), of a dark-capability of progressing through cell division. We derived the action spectra by irradiating E. gracilis with monochromatic light for 3h at around the 26th hour; the action maxima occurred at 380, 450-460, 480, 610, 640, 660, 680, and 740nm. Except for the maximum at 450-460nm, which was always a major maximum, the maxima greatly depended on the red (R)/far-red (FR) ratio of the prior LL. The high R/FR ratio resulted in a dominant major peak at 640nm and minor peaks at 480 and 680nm, whereas the low ratio resulted in dominant major peaks at 610 and 740nm and minor peaks at 380 and 660nm; the critical fluence was minimally about 60mmolm(-2). These HIRs resulted from the accumulation of corresponding low-fluence responses (LFRs) because we found that repetition of a 3-min light/dark cycle, with critical fluences of 1mmolm(-2), lasting for 3h resulted in the same photoinduction as the continuous 3-h irradiation. Moreover, these LFRs expressed photoreversibility. Thus, photoperiodic photoinduction involves Euglena-phytochrome (640 and 740nm) and blue photoreceptor (460nm). Although 380, 480, 610, 660, and 680nm may also represent Euglena-phytochrome, a definite conclusion awaits further study.

Absorption and emission spectroscopic characterization of blue-light receptor Slr1694 from Synechocystis sp. PCC6803

The BLUF protein Slr1694 from the cyanobacterium Synechocystis sp. PCC6803 is characterized by absorption and emission spectroscopy. Slr1694 expressed from E. coli which non-covalently binds FAD, FMN, and riboflavin (called Slr1694(I)), and reconstituted Slr1694 which dominantly contains FAD (called Slr1694(II)) are investigated. The receptor conformation of Slr1694 (dark adapted form Slr1694(r)) is transformed to the putative signalling state (light adapted form Slr1694(s)) with red-shifted absorption and decreased fluorescence efficiency by blue-light excitation. In the dark at 22 degrees C, the signalling state recovers back to the initial receptor state with a time constants of about 14.2s for Slr1694(I) and 17s for Slr1694(II). Quantum yields of signalling state formation of approximately 0.63+/-0.07 for both Slr1694(I) and Slr1694(II) were determined by transient transmission measurements and intensity dependent steady-state transmission measurements. Extended blue-light excitation causes some bound flavin conversion to the hydroquinone form and some photo-degradation, both with low quantum efficiency. The flavin-hydroquinone re-oxidizes slowly back (time constant 5-9 min) to the initial flavoquinone form in the dark. A photo-cycle dynamics scheme is presented.

Photosensitive signal transduction induces membrane hyperpolarization in Paramecium bursaria

The protozoan ciliate Paramecium bursaria exhibits membrane hyperpolarization in response to photostimulation, accompanied with an increased swimming speed. The external addition of cyclic nucleotide phosphodiesterase (PDE) inhibitors, either theophylline (1,3-dimethylxanthine) or 3-isobutyl-1-methylxanthin (IBMX), increased in both amplitudes of the membrane hyperpolarization and the increase in swimming speed. Moreover, the addition of membrane permeable cyclic nucleotide analogs, either 8-bromo-adenosine 3',5'-cyclic monophosphate (Br-cAMP) or 8-Br-guanosine 3',5'-cyclic monophosphate (Br-cGMP), increased these amplitudes. On the other hand, the addition of l-cis-diltiazem, known to block the conductance of cyclic nucleotide-gated channels, partially decreased both amplitudes of the membrane hyperpolarization and the increase in swimming speed. An enzyme immunoassay of cellular cyclic nucleotide contents showed that photostimulation induced a rapid increase in adenosine 3',5'-cyclic monophosphate (cAMP), but little increase in guanosine 3',5'-cyclic monophosphate (cGMP), raising the possibility that a rapid increase in cAMP mediates the light-induced hyperpolarization in P. bursaria.

Photodynamics of the small BLUF protein BlrB from Rhodobacter sphaeroides

The BLUF protein BlrB from the non-sulphur anoxyphototrophic purple bacterium Rhodobacter sphaeroides is characterized by absorption and emission spectroscopy. BlrB expressed from E. coli binding FAD, FMN, and riboflavin (called BrlB(I)) and recombinant BlrB containing only FAD (called BlrB(II)) are investigated. The dark-adapted proteins exist in two different receptor conformations (receptor states) with different sub-nanosecond fluorescence lifetimes (BLUF(r,f) and BLUF(r,sl)). Some of the flavin-cofactor (ca. 8%) is unbound in thermodynamic equilibrium with the bound cofactor. The two receptor conformations are transformed to putative signalling states (BLUF(s,f) and BLUF(s,sl)) of decreased fluorescence efficiency and shortened fluorescence lifetime by blue-light excitation. In the dark at room temperature both signalling states recover back to the initial receptor states with a time constant of about 2s. Quantum yields of signalling state formation of about 90% for BlrB(II) and about 40% for BlrB(I) were determined by intensity dependent transmission measurements. Extended blue-light excitation causes unbound flavin degradation (formation of lumichrome and lumiflavin-derivatives) and bound cofactor conversion to the semiquinone form. The flavin-semiquinone further reduces and the reduced flavin re-oxidizes back in the dark. A photo-dynamics scheme is presented and relevant quantum efficiencies and time constants are determined.

Rhodopsin-mediated photoreception in cryptophyte flagellates

We show that phototaxis in cryptophytes is likely mediated by a two-rhodopsin-based photosensory mechanism similar to that recently demonstrated in the green alga Chlamydomonas reinhardtii, and for the first time, to our knowledge, report spectroscopic and charge movement properties of cryptophyte algal rhodopsins. The marine cryptophyte Guillardia theta exhibits positive phototaxis with maximum sensitivity at 450 nm and a secondary band above 500 nm. Variability of the relative sensitivities at these wavelengths and light-dependent inhibition of phototaxis in both bands by hydroxylamine suggest the involvement of two rhodopsin photoreceptors. In the related freshwater cryptophyte Cryptomonas sp. two photoreceptor currents similar to those mediated by the two sensory rhodopsins in green algae were recorded. Two cDNA sequences from G. theta and one from Cryptomonas encoding proteins homologous to type 1 opsins were identified. The photochemical reaction cycle of one Escherichia-coli-expressed rhodopsin from G. theta (GtR1) involves K-, M-, and O-like intermediates with relatively slow (approximately 80 ms) turnover time. GtR1 shows lack of light-driven proton pumping activity in E. coli cells, although carboxylated residues are at the positions of the Schiff base proton acceptor and donor as in proton pumping rhodopsins. The absorption spectrum, corresponding to the long-wavelength band of phototaxis sensitivity, makes this pigment a candidate for one of the G. theta sensory rhodopsins. A second rhodopsin from G. theta (GtR2) and the one from Cryptomonas have noncarboxylated residues at the donor position as in known sensory rhodopsins.

Kinetic analysis of the activation of photoactivated adenylyl cyclase (PAC), a blue-light receptor for photomovements of Euglena

Photoactivated adenylyl cyclase (PAC) was first purified from a photosensing organelle (the paraflagellar body) of the unicellular flagellate Euglena gracilis, and is regarded as the photoreceptor for the step-up photophobic response. Here, we report the kinetic properties of photoactivation of PAC and a change in intracellular cAMP levels upon blue light irradiation. Activation of PAC was dependent both on photon fluence rate and duration of irradiation, between which reciprocity held well in the range of 2--50 micromol m(-2) s(-1)(total fluence of 1200 micromol m(-2)). Intermittent irradiation also caused activation of PAC in a photon fluence-dependent manner irrespective of cycle periods. Wavelength dependency of PAC activation showed prominent peaks in the UV-B/C, UV-A and blue regions of the spectrum. The time course of the changes in intracellular cAMP levels corresponded well with that of the step-up photophobic response. From this and the kinetic properties of PAC photoactivation, we concluded that an increase in intracellular cAMP levels evoked by photoactivation of PAC is a key event of the step-up photophobic response.

Photoactivated adenylyl cyclase (PAC) genes in the flagellate Euglena gracilis mutant strains

The unicellular, green flagellate wild-type Euglena gracilis(strain Z) and its colorless phototaxis-mutant strains as well as the non-photosynthetic close relative, Astasia longa, possess several genes of the photoactivated adenylyl cyclase (PAC) family. The corresponding gene products were found to be responsible for step-up (but not step-down) photophobic responses as well as both positive and negative phototaxis. The proteins consist of two PACalpha(M(r) 105 kDa) and two PACbeta(90 kDa) subunits. While the proteins were first believed all to be located in the paraxonemal body (PAB), confocal microscopy revealed that Astasia longa as well as some of the mutant strains do not contain a PAB. Immunofluorescence using PAC antibodies showed that the PAC proteins are also located along the total length of the flagellum at least in some of the strains. In order to determine if the genes responsible for the PAC proteins in the PAB and flagella are identical, sequences of all PAC proteins were analyzed in the Euglena and Astasia strains studied for PAC protein location. Full sequence analysis using PCR and 3' and 5' RACE indicated a substantial divergence between strains with a homology between strains of between 45 and 100%. Sequence alignment and sequence tree construction for the main functional groups (BLUF domain, which binds FAD, and adenylyl cyclase) showed that the pacalpha and the pacbeta gene products form clusters each with some of the mutants being closely related while others show a substantial degree of genetic diversity. The conclusion of these results is that there is a family of very dissimilar PAC proteins located in the PAB and the flagellum where they serve different functions in phototaxis and step-up photophobic reactions.

Light matters: phototaxis and signal transduction in unicellular cyanobacteria

Many photosynthetic microorganisms have evolved the ability to sense light quality and/or quantity and can steer themselves into optimal conditions within the environment. Phototaxis and gliding motility in unicellular cyanobacteria require type IV pili, which are multifunctional cell surface appendages. Screens for cells exhibiting aberrant motility uncovered several non-motile mutants as well as some that had lost positive phototaxis (consequently, they were negatively phototactic). Several negatively phototactic mutants mapped to the tax1 locus, which contains five chemotaxis-like genes. This locus includes a gene that encodes a putative photoreceptor (TaxD1) for positive phototaxis. A second chemotaxis-like cluster (tax3 locus) appears to be involved in pilus biogenesis. The biosynthesis and regulation of type IV pilus-based motility as well as the communication between the pilus motor and photosensory molecules appear to be complex and tightly regulated. Furthermore, the discovery that cyclic AMP and novel gene products are necessary for phototaxis/motility suggests that there might be additional levels of communication and signal processing.