A set of 62 genes that encode the entire peptidase complement of Synechocystis sp. PCC 6803 has been identified in the genome database of that cyanobacterium. Sequence comparisons with the Arabidopsis genome uncovered the presumably homologous chloroplast components inherited from their cyanobacterial ancestor. A systematic gene disruption approach was chosen to individually inactivate, by customary transformation strategies, the majority of the cyanobacterial genes encoding peptidase subunits that are related to chloroplast enzymes. This allowed classification of the peptidases that are required for cell viability or are involved in specific stress responses. The comparative analysis between Synechocystis and Arabidopsis chloroplast peptidases showed that: (1) homologous enzymes that arose by gene duplications in cyanobacteria are functionally diverse and frequently do not complement each other, (2) the chloroplast appears to house a number of distinct peptidase polypeptide chains of cyanobacterial origin (49) which is comparable with a cyanobacterial cell (62) and (3) the peptidase complement in plastids results from a combination of the loss of some cyanobacterial peptidases and the gain or diversification of subclasses of peptidases. This reorganization in the pattern of proteolytic enzymes may reflect distinct environmental and physiological changes between prokaryotic and organellar systems.

Cyanobacteria are the only prokaryotes that perform oxygenic photosynthesis and are thought to be ancestors to plant chloroplasts. Like chloroplasts, cyanobacteria possess a diverse array of proteolytic enzymes, with one of the most prominent being the ATP-dependent Ser-type Clp protease. The model Clp protease in Escherichia coli consists of a single ClpP proteolytic core flanked on one or both ends by a HSP100 chaperone partner. In comparison, cyanobacteria have multiple ClpP paralogs plus a ClpP variant (ClpR), which lacks the catalytic triad typical of Ser-type proteases. In this study, we reveal that two distinct soluble Clp proteases exist in the unicellular cyanobacterium Synechococcus elongatus. Each protease consists of a unique proteolytic core comprised of two separate Clp subunits, one with ClpP1 and ClpP2, the other with ClpP3 and ClpR. Each core also associates with a particular HSP100 chaperone partner, ClpC in the case of the ClpP3/R core, and ClpX for the ClpP1/P2 core. The two adaptor proteins, ClpS1 and ClpS2 also interact with the ClpC chaperone protein, likely increasing the range of protein substrates targeted by the Clp protease in cyanobacteria. We also reveal the possible existence of a third Clp protease in Synechococcus, one which associates with the internal membrane network. Altogether, we show that presence of several distinctive Clp proteases in cyanobacteria, a feature which contrasts from that in most other organisms.

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The sll1703 gene, encoding an Arabidopsis homologue of the thylakoid membrane-associated SppA peptidase, was inactivated by interposon mutagenesis in Synechocystis sp. strain PCC 6803. Upon acclimation from a light intensity of 50 to 150 E m ؊2 s ؊1 , the mutant preserved most of its phycobilisome content, whereas the wild-type strain developed a bleaching phenotype due to the loss of about 40% of its phycobiliproteins. Using in vivo and in vitro experiments, we demonstrate that the ⌬sppA1 strain does not undergo the cleavage of the L R 33 and L CM 99 linker proteins that develops in the wild type exposed to increasing light intensities. We conclude that a major contribution to light acclimation under a moderate light regime in cyanobacteria originates from an SppA1-mediated cleavage of phycobilisome linker proteins. Together with changes in gene expression of the major phycobiliproteins, it contributes an additional mechanism aimed at reducing the content in phycobilisome antennae upon acclimation to a higher light intensity.