Characterization of psal and psaL Mutants of Synechococcus sp. Strain PCC 7002: A New Model for State Transitions in Cyanobacteria

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Abstract
The psal 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 psal occurring upstream and being divergently transcribed from the psaL gene. Mutants lacking Psal or PsaL were generated by interposon mutagenesis and characterized physiologically and biochemically. Mutant strains PR6307 (Δpsal−), PR6308 (psal) 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 PS I complexes from the psal mutants, but not the psaL mutant, could be isolated as trimers when n‐do‐decyl β‐D‐maltoside was used. Compositional analyses of the mutant PS I complexes indicate that the presence of PsaL is required for trimer formation or stabilization and that Psal plays a role in stabilizing the binding of both PsaL and PsaM to the PS I complex. Strain PR6309 (psaL−) was capable of performing a state 2 to state 1 transition approximately three times more rapidly than the wild type. Because the monomeric PS I complexes of this mutant should be capable of diffusing more rapidly than trimeric complexes, these data suggest that PS I complexes rather than phycobilisomes might move during state transitions. A “mobile‐PS I” model for state transitions that incorporates these ideas is discussed.