Continuous chlorophyll degradation accompanied by chlorophyllide and phytol reutilization for chlorophyll synthesis in Synechocystis sp PCC 6803

Chlorophyll synthesis and degradation were analyzed in the cyanobacterium Synechocystis sp. PCC 6803 by incubating cells in the presence of C-13-labeled glucose or N-15-containing salts. Upon mass spectral analysis of chlorophyll isolated from cells grown in the presence of C-13-glucose for different time periods, four chlorophyll pools were detected that differed markedly in the amount of C-13 incorporated into the porphyrin (Per) and phytol (Phy) moieties of the molecule. These four pools represent (i) unlabeled chlorophyll ((12)Por(12)Phy), (ii) C-13-labeled chlorophyll ((13)Por (13)Phy), and (iii, iv) chlorophyll, in which either the porphyrin or the phytol moiety was C-13-labeled, whereas the other constituent of the molecule remained unlabeled ((13)Por(12)Phy and (12)Por (13)Phy). The kinetics of (12)Por(12)Phy disappearance, presumably due to chlorophyll deesterification, and of (13)Por(12)Phy, (12)Por(13)Phy, and (13)Por(13)Phy accumulation due to chlorophyll synthesis provided evidence for continuous chlorophyll turnover in Synechocystis cells. The loss of (12)Por(12)Phy was three-fold faster in a photosystem I-less strain than in a photosystein II-less strain and was accelerated in wild-type cells upon exposure to strong light. These data suggest that most chlorophyll appears to be deesterified 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 (13)Por(12)Phy and (12)Por(13)Phy chlorophyll pools. The phytol kinase, Slr1652, plays a significant but not absolutely critical role in this recycling process. (C) 2007 Elsevier B.V. All rights reserved.