MUTATIONAL ANALYSIS OF THE PSBL PROTEIN OF PHOTOSYSTEM-II IN THE CYANOBACTERIUM SYNECHOCYSTIS SP PCC-6803

The psbL gene is a member of the psbEFLJ gene cluster in the cyanobact erium Synechocystis sp. PCC 6803 and higher plants. psbL, a 4.5 kDa pr otein encoded by this gene, is a component of the photosystem II compl ex. The amino acid sequence of this protein indicates that it has a si ngle membrane-spanning alpha-helical domain. We have used a targeted m utagenesis technique to delete the coding region of the psbL gene in S ynechocystis 6803. The resultant mutant strain T345 did not show any P SII-mediated oxygen evolution activity and, as a result, could not gro w under photoautotrophic conditions. However, it had normal PSI activi ty. The chlorophyll to phycobilin ratio in the T345 cells was signific antly lower than that in the wild type cells. Fluorescence emission sp ectra (77 K) of the mutant cells showed the absence of a 695 nm band t hat usually originates from the PSII complex. Binding assays with radi oactive diuron demonstrated that the mutant cells did not have any her bicide binding activity. However, immunostaining experiments showed th at both the D1 (the herbicide binding protein) and the D2 proteins of the PSII reaction center were present at > 25% of their normal levels in the thylakoid membranes of the T345 mutant cells. Our data indicate that the PsbL protein is essential for the normal functioning of PSII .