Structure and function of the photoreceptor stentorins in Stentor coeruleus. II. Primary photoprocess and picosecond time-resolved fluorescence

Picosecond decay kinetics and quantum yield of fluorescence of the photoactive yellow protein from the halophilic purple phototrophic bacterium, Ectothiorhodospira halophila

The photoactive yellow protein (PYP) has been previously shown to be partially bleached and red shifted (in less than 10 ns) by a pulse of laser excitation at the wavelength maximum (445 nm), to further bleach (k = 7.5 x 10(3) s(-1)), and then to slowly recover in the dark (k = 2.6 s(-1)) (Meyer, T. E., G. Tollin, J. H. Hazzard, and M. A. Cusanovich. 1989. Biophys. J. 56:559-564). The quantum yield for the formation of the fully bleached form was found to be 0.64. We have now shown that the yellow protein is weakly fluorescent with an emission maximum at 495 nm (which mirrors excitation at 445 nm) and a fluorescence quantum yield of 1.4 x 10(-3). Measurement of the picosecond kinetics of the fluorescence decay shows that approximately 90% of the emission occurs with a lifetime of 12 ps. This is in good agreement with the quantum yield determination, which suggests that a single quenching process (presumably the photochemical event) is primarily responsible for the excited state decay. The lifetime of the excited state of PYP is remarkably similar to that for the rise of the first photochemical intermediate of bacteriorhodopsin, and underscores the fundamental similarity in their photocycles despite a lack of structural relationship.

Photophysics and Multifunctionality of Hypericin-Like Pigments in Heterotrich Ciliates: A Phylogenetic Perspective

In this paper, we review the literature and present some new data to examine the occurrence and photophysics of the diverse hypericin-like chromophores in heterotrichs, the photoresponses of the cells, the various roles of the pigments and the taxa that might be studied to advance our understanding of these pigments. Hypericin-like chromophores are known chemically and spectrally so far only from the stentorids and Fabrea, the latter now seen to be sister to stentorids in the phylogenetic tree. For three hypericin-like pigments, the structures are known but these probably do not account for all the colors seen in stentorids. At least eight physiological groups of Stentor exist depending on pigment color and presence/absence of zoochlorellae, and some species can be bleached, leading to many opportunities for comparison of pigment chemistry and cell behavior. Several different responses to light are exhibited among heterotrichs, sometimes by the same cell; in particular, cells with algal symbionts are photophilic in contrast to the well-studied sciaphilous (shade-loving) species. Hypericin-like pigments are involved in some well-known photophobic reactions but other pigments (rhodopsin and flavins) are also involved in photoresponses in heterotrichs and other protists. The best characterized role of hypericin-like pigments in heterotrichs is in photoresponses and they have at least twice evolved a role as photoreceptors. However, hypericin and hypericin-like pigments in diverse organisms more commonly serve as predator defense and the pigments are multifunctional in heterotrichs. A direct role for the pigments in UV protection is possible but evidence is equivocal. New observations are presented on a folliculinid from deep water, including physical characterization of its hypericin-like pigment and its phylogenetic position based on SSU rRNA sequences. The photophysics of hypericin and hypericin-like pigments is reviewed. Particular attention is given to how their excited-state properties are modified by the environment. Dramatic changes in excited-state behavior are observed as hypericin is moved from the homogeneous environment of organic solvents to the much more structured surroundings provided by the complexes it forms with proteins. Among these complexes, it is useful to consider the differences between environments where hypericin is not found naturally and those where it is, notably, for example, in heterotrichs. It is clear that interaction with a protein modifies the photophysics of hypericin and understanding the molecular basis of this interaction is one of the outstanding problems in elucidating the function of hypericin and hypericin-like chromophores.

Photoreception and photomovements of microorganisms

Many freely motile microorganisms can perceive and transduce external photic stimuli to the motor apparatus, eventually moving, by means of various behavioural strategies, into environments in which the illumination conditions are the most favourable for their life. In different microorganisms, a wide range of chromophores operate as light detectors, each of them set in a special molecular pocket that, in its turn, can be linked to another component of the transduction chain. The diverse photosensors are organized in special (and in many cases dedicated) photoreceptor units or subcellular organelles. The main molecular mechanisms connecting the early event of photon absorption to the formation of the signalling state down to the dark steps of the transduction chain are discussed in a selected number of case examples. The possible importance of an intensive multidisciplinary approach to these problems in an evolutionary perspective is finally briefly outlined.

Initial spectroscopic characterization of the ciliate photoreceptor stentorin

Stentorin serves as the primary photosensor in the single cell ciliate, Stentor coeruleus, for its photophobic and phototactic response to light of visible wavelengths. We separated two subunits, stentorin-2A and -2B, from the previous stentorin complex ('stentorin-2') of greater than half a million molecular mass isolated from the photoreceptor organelle (pigment granule). Stentorin-2B bears the chromophore covalently linked to an approx. 50 kDa apoprotein, as determined by SDS-urea-PAGE. Partial amino acid sequences were obtained from this 50 kDa subunit. Its visible and CD spectra were found to be similar to those of stentorin-2. The steady-state and time-resolved fluorescence spectra of stentorin-2B, in H2O and D2O buffers, were also similar to those of stentorin-2. This suggests that the 50 kDa subunit retains the spectral integrity and primary photoreactivity of the stentorin-complex. The picosecond time-resolved fluorescence study revealed that the short picosecond emission component (tau F approximately equal to 8-10 ps) was the predominant emitting species in stentorin-2B and -2, followed by longer decaying species. No deuterium solvent effect was seen in this fast-decaying species. The possible mechanism for the primary photoreaction appears to involve electron transfer coupled with proton transfer.

Spectroscopic and photoacoustic studies of hypericin embedded in liposomes as a photoreceptor model

In photoresponsive ciliates, like Blepharisma japonicum and Stentor coeruleus, the photoreceptor pigments responsible for photomotile reactions are hypericin-type chromophores packed in highly osmiophilic subpellicular granules. Lipopsomes loaded with hypericin can constitute a simple model system, appropriate for understanding the primary light-induced molecular events triggering the sensory chain in these microorganisms. Optical absorption, steady-state and time-resolved fluorescence and pulsed photoacoustic calorimetry have been used to measure spectral distributions, fluorescence lifetimes, radiative and radiationless transition quantum yields of hypericin when assembled into egg L-alpha-phosphatidylcholine liposomes. With respect to hypericin ethanol solutions, both absorption and fluorescence maxima are 5 nm red shifted when the pigment is inserted into the lipidic microenvironment, regardless of the hypericin local concentration. Increasing by 100 times the hypericin local concentration decreases the relative fluorescence quantum yield by a factor of around 150 and the fraction of thermally released energy, conversely, increases from 0.6 to 0.9. From the analysis of fluorescence lifetimes and their relative amplitudes it appears that a subnanosecond living component is predominant at the highest hypericin local concentrations.

Time-resolved fluorescence spectroscopy and photolysis of the photoreceptor blepharismin

Blepharismin is the photoreceptor for the photophobic response in the ciliate Blepharisma japonicum (Scevoli, P., Bisi, F., Colombetti, G., Ghetti, F., Lenci, F., and Passarelli, V. (1987) J. Photochem. Photobiol.: B. Biol. 1, 75-84; Lenci, F., Ghetti, F., Gioffre, D., Heelis, P.F., Thomas, B., Phillips, G.O., and Song, P.-S. (1989) J. Photochem. Photobiol.: B. Biol. 3, 449-453). Blepharismin was solubilized from the red cells with 2% n-octylglucopyranoside. A crude pigment-protein preparation was then successively subjected to Bio-Gel A1.5 filtration, FPLC/hydroxyapatite and FPLC/DEAE ion-exchange chromatography. At least two spectrally distinct forms of blepharismin, with the respective absorbance maxima at 597 +/- 1 and 601 +/- 1 nm, were resolved. The steady state fluorescence emission maxima were at 602.5 and 617.5 nm, respectively. The fluorescence decay curves for these pigments were non-exponential. The major component possesses relatively short fluorescence lifetime (200-500 ps) for the former, according to a global analysis. This analysis suggests that the excited state of the shorter wavelength-absorbing form of blepharismin undergoes primary photoprocess faster than that of the free parental chromophore hypericin. Photolysis of blepharismin in solution yielded a irreversible product, accompanied by a 10-12 nm bathochromic shift of the absorbance maximum. However, the mechanistic nature of the time-resolved fluorescence and the photochemistry of blepharismin remains to be elucidated.

Subnanosecond single photon timing measurements using a pulsed diode-laser

The convenient and inexpensive use of a pulsed diode-laser (Hamamatsu Photonics PLP-01 660 nm) is demonstrated as a low cost alternative to a standard pulsed laser or gas discharge flash system in a commercial time-correlated single photon counting instrument. Fluorescence lifetimes of compounds of photobiological interest such as phytochrome, chlorophyll a, 1,1'-diethyl-4,4' carbocyanine iodide (DCI/cryptocyanin),5,10,15,20-tetra(p-phenyl) porphyrin and stentorin I are presented using the pulsed diode-laser source.

Structure and function of the photoreceptor stentorins in Stentor coeruleus. I. Partial characterization of the photoreceptor organelle and stentorins

The unicellular ciliary protozoan, Stentor coeruleus, exhibits photophobic and phototactic responses to visible light stimuli. The pigment granule contains the photoreceptor chromoproteins (stentorins). Stentorin localized in the pigment granules of the cell serves as the primary photoreceptor for the photophobic and phototactic responses in this organism. An initial characterization of the pigment granules has been described in terms of size, absorbance spectra and ATPase activity. Two forms of the stentorin pigments have been isolated from the pigment granules. Stentorin I has an apparent molecular weight of 68,600 and 52,000 by SDS-PAGE (at 10 and 13% gel, respectively) or 102,000 by steric exclusion HPLC, whereas stentorin II is a larger molecular assembly probably composed of several proteins (mol. wt. greater than 500,000). Stentorin I is composed of at least two heterologous subunits corresponding to apparent mol. wts. of 46,000 (fluorescent, Coomassie blue negative) and 52,000 (fluorescent, Coomassie blue positive) on SDS-PAGE (13% gel). However, these values were found to be strongly dependent on the degree of crosslinking in the acrylamide gel. Stentorin II appears to be the primary photoreceptor whose absorption and fluorescence properties are consistent with the action spectra for the photoresponses of the ciliate to visible light.