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The kinetics of the photoreceptor potential of phototaxis in biflagellated green alga Haematococcus pluvialis in response to a 10-ns laser pulse of three wavelengths (465, 550, and 590 nm) were measured in single cells with 30 mus time resolution. The rise and the decay of photoinduced potential are both at least biphasic. The first component of the rise is very stable and has no measurable (<30 mus) time delay. The second component is triggered after a 120-400-mus lag period, depending on flash intensity. Its appearance is sensitive to the physiological state of the cell and the amplitude can be increased by phototactically ineffective red background illumination. The electrical generators for both components are localized in the same region of the cell membrane (on the stigma-bearing side) and these components have the same depolarizing sign. The results indicate that the photoreceptor potential in phototaxis comprises two components, which could be interpreted as light-induced charge movement within the photoreceptor molecules and changes in ion permeability of the cell membrane.

In aqueous suspensions of purple membranes (pH 10.2, 0.4 M KCl)

It has been found that at low temperatures (77K-153K) a long-lived (at these temperatures) singlet ESR signal induced by intensive light appears in etiolated leaves of plants and in model systems including both the monomeric and aggregated protochlorophyll.Comparison of the results of ESR, fluorescence and absorption spectra measurements made it possible to suggest that at the initial stages of the protochlorophyll(ide) photoreduction process at least two paramagnetic non-fluorescent intermediates are formed, one of which seems to be identical to the previously found intermediate with absorption maximum at 690 nm. On the strength of the obtained results a conclusion can be drawn that photoreduction of the semi-isolated double-c=c-bond of the chlorophyll precursor molecule in etiolated leaves and in model systems is actualized via at least two stages of free radicals formation. A scheme of the primary reactions of chlorophyllide biosynthesis has been proposed.

At 90 K the photoproduct of the primary light reaction of trans-bacteriorhodopsin, the bathoproduct K', consists of a mixture of at least three spectrally different species, K;, K;, and K;,,, having maxima i n the difference absorption spectra at 645, 635 and 625 nm, respectivcly. The bathoproducts differ in their long wavelength absorption bands and in their rate constants for photoconversion to tram-bacteriorhodopsin under far red light irradiation (A > 720 nm). The bathoproducts are formed from different precursors+onformers of trans-bacteriorhodopsin, which are stable at 90 K, but are in equilibrium with each other at 213 K. We suggest that the bathoproducts may initiate parallel conversion cycles of trans-bacteriorhodopsin at low temperatures. The primary photoreaction of 13-cis-bacteriorhodopsin also yields three bathoproducts, K;, K;, and K;,,, having maxima in the difference absorption spectra at 615, 605 and 595 nm, respectively.

This review summarizes contemporary data on structure and function of photoactive pigment--enzyme complexes of the chlorophyll precursor that undergoes photochemical transformation to chlorophyllide. The properties and functions of the complex and its principal components are considered including the pigment (protochlorophyllide), the hydrogen donor (NADPH), and the photoenzyme protochlorophyllide oxidoreductase (POR) that catalyzes the photochemical production of chlorophyllide. Chemical variants of the chlorophyll precursor are described (protochlorophyllide, protochlorophyll, and their mono- and divinyl forms). The nature and photochemical activity of spectrally distinct native protochlorophyllide forms are discussed. Data are presented on structural organization of the photoenzyme POR, its substrate specificity, localization in etioplasts, and heterogeneity. The significance of different POR forms (PORA, PORB, and PORC) in adaptation of chlorophyll biosynthesis to various illumination conditions is considered. Attention is paid to structural and functional interactions of three main constituents of the photoactive complex and to possible existence of additional components associated with the pigment-enzyme complex. Historical aspects of the problem and the prospects of further investigations are outlined.

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Illumination of etiolated maize leaves with low-intensity light produces a chlorophyll/pheophytin-containing complex. The complex contains two native chlorophyll forms Chl 671/668 and Chl 675/668 as well as pheophytin Pheo 679/675 (with chlorophyll/pheophytin ratio of 2/1). The complex is formed in the course of two successive reactions: reaction of protochlorophyllide Pchlde 655/650 photoreduction resulted in chlorophyllide Chlde 684/676 formation, and the subsequent dark reaction of Chlde 684/676 involving Mg substitution by H2 in pigment chromophore and pigment esterification by phytol. Out data show that the reaction leading to chlorophyll/pheophytin-containing complex formation is not destructive. The reaction is in fact biosynthetic, and is competitive with the known reactions of biosynthesis of the bulk of chlorophyll molecules. The relationship between chlorophyll and pheophytin biosynthesis reactions is controlled by temperature, light intensity and exposure duration.The native complex containing pheophytin a and chlorophyll a is supposed to be a direct precursor of the PS II reaction centre in plant leaves.