CHARACTERIZATION OF PSAI AND PSAL MUTANTS OF SYNECHOCOCCUS SP STRAIN PCC-7002 - A NEW MODEL FOR STATE TRANSITIONS IN CYANOBACTERIA

The psaI and psaL genes were characterized from the cyanobacterium Syn echococcus sp. strain PCC 7002. The gene organization was different fr om that reported for other cyanobacteria with psaI occurring upstream and being divergently transcribed from the psaL gene, Mutants lacking PsaI or PsaL were generated by interposon mutagenesis and characterize d physiologically and biochemically. Mutant strains PR6307 (Delta psaI ), PR6308 (psaI(-)) and PR6309 (psaL(-)) had doubling times similar to that of the wild type under both high- and low-intensity white light, but all grew more slowly than the wild type in green light. Only mono meric photosystem I (PS I) complexes could be isolated from each mutan t strain when Triton X-100 was used to solubilize thylakoid membranes; however, approximately 10% of the PS I complexes from the psaI mutant s, but not the psaL mutant, could be isolated as trimers when n-dodecy l beta-D-maltoside was used. Compositional analyses of the mutant PS I complexes indicate that the presence of PsaL is required for trimer f ormation or stabilization and that PsaI plays a role in stabilizing th e binding of both PsaL and PsaM to the PS I complex. Strain PR6309 (ps aL(-)) was capable of performing a state 2 to state 1 transition appro ximately three times more rapidly than the wild type. Because the mono meric PS I complexes of this mutant should be capable of diffusing mor e rapidly than trimeric complexes, these data suggest that PS I comple xes rather than phycobilisomes might move during state transitions. A ''mobile-PS I'' model for state transitions that incorporates these id eas is discussed.