D1 POLYPEPTIDE DEGRADATION MAY REGULATE PSBA GENE-EXPRESSION AT TRANSCRIPTIONAL AND TRANSLATIONAL LEVELS IN SYNECHOCYSTIS SP PCC-6803

Light has been suggested to regulate both synthesis and degradation of the Photosystem II (PS II) reaction centre polypeptide D1, encoded by the psbA gene. The modified degradation rate of the D1 polypeptide in site-directed Synechocystis sp PCC 6803 D1 mutants CA1 [del(E242-E244 );Q241H], E243K and E229D has provided a tool to determine whether the rate of D1 polypeptide synthesis is directly regulated by light-inten sity-related factors or by a control mechanism mediated by light-depen dent degradation of the D1 polypeptide. In vivo accumulation of [S-35] methionine into the D1 polypeptide was found to correlate with D1 pol ypeptide degradation rather than with incident irradiance. This sugges ts that the degradation rate of the D1 polypeptide regulates its own s ynthesis at translational level. Furthermore, several fold differences in the psbA mRNA levels were measured between D1 mutant strains, indi cating that the psbA gene transcription is not solely under light cont rol.