UV-induced phycobilisome dismantling in the marine picocyanobacterium Synechococcus sp. WH8102.

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.