Characterization of psal 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 Synechococcus sp. strain PCC 7002. The gene organization was different from 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 characterized physiologically and biochemically. Mutant strains PR6307 (ΔpsaI), 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 monomeric photosystem I (PS I) complexes could be isolated from each mutant strain when Triton X-100 was used to solubilize thylakoid membranes; however, approximately 10% of the PSI complexes from the psaI mutants, but not the psaL mutant, could be isolated as trimers when n-dodecyl β-D-maltoside was used. Compositional analyses of the mutant PSI complexes indicate that the presence of PsaL is required for trimer formation or stabilization and that PsaI plays a role in stabilizing the binding of both PsaL and PsaM to the PSI complex. Strain PR6309 (psaL) was capable of performing a state 2 to state I transition approximately three times more rapidly than the wild type. Because the monomeric PSI complexes of this mutant should be capable of diffusing more rapidly than trimeric complexes, these data suggest that PSI complexes rather than phycobilisomes might move during state transitions. A 'mobile-PS I' model for state transitions that incorporates these ideas is discussed.