In oxygenic photosynthesis, chlorophyll plays a crucial rule in light harvesting as well as in transformation of absorbed light energy to chemical energy. However, the combination of chlorophyll and light is both a blessing and a curse for photosynthetic organisms. Whereas both are required for photosynthesis, chlorophyll excited by light can be converted to the triplet state, which in the presence of oxygen may lead to the formation of potentially damaging singlet oxygen ( 1 O 2 ) (1). The photodestructive potential of chlorophyll is greatly reduced if the chlorophyll is adjacent to carotenoids, which are efficient quenchers of both triplet chlorophyll and 1 O 2 (2). In vivo this is accomplished through pigment-binding proteins, which provide adjacency between chlorophylls and carotenoids and which allow efficient transfer of excitations eventually to the reaction center chlorophyll. In higher plants, the major chlorophyllbinding proteins are those that are part of the core complex of photosystem (PS) 1 I and PS II as well as the light-harvesting antenna complexes, which are encoded by a multigene family of cab genes (3). Most of the cab gene family members code for chlorophyll a/b-binding proteins with three transmembrane helices, of which the sequences of the first and third membrane-spanning regions are similar and where chlorophyllbinding sites are highly conserved. Other members of the cab gene family in plants are psbS, the product of which is predicted to have four membrane-spanning regions (4) and affects nonphotochemical quenching (5), as well as genes for singlehelix (6) and two-helix (7) members.In contrast to plants, cyanobacteria use phycobilisomes as the major light-harvesting apparatus, and they do not contain multihelix Cab proteins (light-harvesting antenna complexes or related chlorophyll a/b-binding peripheral antenna proteins). However, numerous genes potentially coding for small proteins with a single membrane-spanning region similar to the first and third transmembrane regions of Cab proteins and with conserved residues involved in chlorophyll binding were found in genomes of many cyanobacteria including prochlorophytes (see Refs. 8 and 9). For example, the Prochlorococcus marinus MED4 genome contains as many as 23 such genes (9). The genome of the cyanobacterium Synechocystis sp. PCC 6803 contains genes for four small Cab-like proteins (SCPs) that have been named ScpB-ScpE (10). In addition, ferrochelatase in cyanobacteria and the chloroplast-targeted isozyme in plants has a ϳ60-residue C-terminal extension (absent in other ferrochelatases) that is similar to the SCPs. For this reason, the C-terminal extension has been named ScpA and this SCP extension does not seem to be necessary for the activity of ferrochelatase in Synechocystis (10). Since one of the conditions under which the small scp genes are expressed is high light intensity, these genes also have been named hli (high lightinducible) by other authors (8,11 Downloaded fromThe function of SCPs is still largely unclear, although...

Chlorophyll synthesis and degradation were analyzed in the cyanobacterium Synechocystis sp. PCC 6803 by incubating cells in the presence of 13C-labeled glucose or 15N-containing salts. Upon mass spectral analysis of chlorophyll isolated from cells grown in the presence of 13C-glucose for different time periods, four chlorophyll pools were detected that differed markedly in the amount of 13C incorporated into the porphyrin (Por) and phytol (Phy) moieties of the molecule. These four pools represent (i) unlabeled chlorophyll (12Por12Phy), (ii) 13C-labeled chlorophyll (13Por13Phy), and (iii, iv) chlorophyll, in which either the porphyrin or the phytol moiety was 13C-labeled, whereas the other constituent of the molecule remained unlabeled (13Por12Phy and 12Por13Phy). The kinetics of 12Por12Phy disappearance, presumably due to chlorophyll de-esterification, and of 13Por12Phy, 12Por13Phy, and 13Por13Phy accumulation due to chlorophyll synthesis provided evidence for continuous chlorophyll turnover in Synechocystis cells. The loss of 12Por12Phy was three-fold faster in a photosystem I-less strain than in a photosystem II-less strain and was accelerated in wild-type cells upon exposure to strong light. These data suggest that most chlorophyll appears to be de-esterified in Synechocystis upon dissociation and repair of damaged photosystem II. A substantial part of chlorophyllide and phytol released upon the de-esterification of chlorophyll can be recycled for the biosynthesis of new chlorophyll molecules contributing to the formation of 13Por12Phy and 12Por13Phy chlorophyll pools. The phytol kinase, Slr1652, plays a significant but not absolutely critical role in this recycling process.

Photosynthetic organisms synthesize chlorophylls, hemes, and bilin pigments via a common tetrapyrrole biosynthetic pathway. This review summarizes current knowledge about the regulation of this pathway in plants, algae, and cyanobacteria. Particular emphasis is placed on the regulation of glutamate-1-semialdehyde formation and on the channelling of protoporphyrin IX into the heme and chlorophyll branches. The potential role of chlorophyll molecules that are not bound to photosynthetic pigment-protein complexes ('free chlorophylls') or of other Mg-containing porphyrins in regulation of tetrapyrrole synthesis is also discussed.

An Arabidopsis thaliana chlorophyll(ide) a oxygenase gene (cao), which is responsible for chlorophyll b synthesis from chlorophyll a, was introduced and expressed in a photosystem I-less strain of the cyanobacterium Synechocystis sp. PCC 6803. In this strain, most chlorophyll is associated with the photosystem II complex. However, when lhcb encoding light-harvesting complex (LHC)II from pea was present in the same strain (lhcb ؉ ͞cao ؉ ), chlorophyll b accumulated in the cell to levels exceeding those of chlorophyll a, although LHCII did not accumulate. In the lhcb ؉ ͞cao ؉ strain, the total amount of chlorophyll, the number of chlorophylls per photosystem II center, and the oxygen-evolving activity on a perchlorophyll basis were similar to those in the photosystem I-less strain. Furthermore, the chlorophyll a͞b ratio of photosystem II core particles (retaining CP47 and CP43) and of whole cells of the lhcb ؉ ͞cao ؉ strain was essentially identical, and PS II activity could be obtained efficiently by chlorophyll b excitation. These data indicate that chlorophyll b functionally substitutes for chlorophyll a in photosystem II. Therefore, the availability of chlorophylls, rather than their binding specificity, may determine which chlorophyll is incorporated at many positions of photosystem II. We propose that the transient presence of a LHCII͞chlorophyll(ide) a oxygenase complex in the lhcb ؉ ͞cao ؉ strain leads to a high abundance of available chlorophyll b that is subsequently incorporated into photosystem II complexes. The apparent LHCII requirement for high chlorophyll(ide) a oxygenase activity may be instrumental to limit the occurrence of chlorophyll b in plants to LHC proteins.

To determine the mechanism of carotenoid-sensitized non-photochemical quenching in cyanobacteria, the kinetics of blue-light-induced quenching and fluorescence spectra were studied in the wild type and mutants of Synechocystis sp. PCC 6803 grown with or without iron. The blue-light-induced quenching was observed in the wild type as well as in mutants lacking PS II or IsiA confirming that neither IsiA nor PS II is required for carotenoid-triggered fluorescence quenching. Both fluorescence at 660 nm (originating from phycobilisomes) and at 681 nm (which, upon 440 nm excitation originates mostly from chlorophyll) was quenched. However, no blue-light-induced changes in the fluorescence yield were observed in the apcE(-) mutant that lacks phycobilisome attachment. The results are interpreted to indicate that interaction of the Slr1963-associated carotenoid with--presumably--allophycocyanin in the phycobilisome core is responsible for non-photochemical energy quenching, and that excitations on chlorophyll in the thylakoid equilibrate sufficiently with excitations on allophycocyanin in wild type to contribute to quenching of chlorophyll fluorescence.