The marine picocyanobacterium Synechococcus sp. WH8102 was submitted to ultraviolet (UV-A and B) radiations and the effects of this stress on reaction center II and phycobilisome integrity were studied using a combination of biochemical, biophysical and molecular biology techniques. Under the UV conditions that were applied (4.3 W m(-2) UV-A and 0.86 W m(-2) UV-B), no significant cell mortality and little chlorophyll degradation occurred during the 5 h time course experiment. However, pulse amplitude modulated (PAM) fluorimetry analyses revealed a rapid photoinactivation of reaction centers II. Indeed, a dramatic decrease of the D1 protein amount was observed, despite a large and rapid increase in the expression level of the psbA gene pool. Our results suggest that D1 protein degradation was accompanied (or followed) by the disruption of the N-terminal domain of the anchor linker polypeptide LCM, which in turn led to the disconnection of the phycobilisome complex from the thylakoid membrane. Furthermore, time course analyses of in vivo fluorescence emission spectra suggested a partial dismantling of phycobilisome rods. This was confirmed by characterization of isolated antenna complexes by SDS-PAGE and immunoblotting analyses which allowed us to locate the disruption site of the rods near the phycoerythrin I-phycoerythrin II junction. In addition, genes encoding phycobilisome components, including alpha-subunits of all phycobiliproteins and phycoerythrin linker polypeptides were all down regulated in response to UV stress. Phycobilisome alteration could be the consequence of direct UV-induced photodamages and/or the result of a protease-mediated process.
In cyanobacteria, the D1 protein of photosystem II (PSII) is encoded by the psbA multigene family. In most freshwater strains, a D1:1 isoform of this protein is exchanged for a D1:2 isoform in response to various stresses, thereby altering PSII photochemistry. To investigate PSII responses to stress in marine Synechococcus, we acclimated cultures of the WH7803 strain to different growth irradiances and then exposed them to high light (HL) or ultraviolet (UV) radiation. Measurement of PSII quantum yield and quantitation of the D1 protein pool showed that HL-acclimated cells were more resistant to UV light than were low light- (LL) or medium light- (ML) acclimated cells. Both UV and HL induced the expression of psbA genes encoding D1:2 and the repression of the psbA gene encoding D1:1. Although three psbA genes encode identical D1:2 isoforms in Synechococcus sp. WH7803, only one was strongly stress responsive in our treatment conditions. Examination of 11 marine Synechococcus genomic sequences identified up to six psbA copies per genome, with always a single gene encoding D1:1. In phylogenetic analyses, marine Synechococcus genes encoding D1:1 clustered together, while the genes encoding D1:2 grouped by genome into subclusters. Moreover, examination of the genomic environment of psbA genes suggests that the D1:2 genes are hotspots for DNA recombination. Collectively, our observations suggest that while all psbA genes follow a concerted evolution within each genome, D1:2 coding genes are subject to intragenome homogenization most probably mediated by gene conversion.
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