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

Abstract

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 (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 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 psaI mutants, but not the psaL mutant, could be isolated as trimers when n-dodecyl beta-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 PsaI 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.

Phycobilisome Diffusion Is Required for Light-State Transitions in Cyanobacteria

“…Phycobilisomes also diffuse more rapidly in PsaL mutants, which is consistent with the idea that state transitions involve phycobilisome mobility (Aspinwall et al, 2004). However, other models that do not involve the movement of phycobilisomes have been proposed (Schluchter et al, 1996;McConnell et al, 2002). Our current results indicate that phycobilisome mobility is critical for state transitions: the same conditions that immobilize phycobilisomes also lock cells into the light state to which they were adapted.…”

“…The mobility of phycobilisomes (Mullineaux et al, 1997) suggests that state transitions may involve the physical decoupling of phycobilisomes from one type of reaction center and their reassociation with the other, as originally proposed by Allen and co-workers (Allen and Holmes, 1986). The transition to State 1 occurs more rapidly in mutants lacking PsaL, the subunit required for trimerization of PSI (Schluchter et al, 1996;Aspinwall et al, 2004). Phycobilisomes also diffuse more rapidly in PsaL mutants, which is consistent with the idea that state transitions involve phycobilisome mobility (Aspinwall et al, 2004).…”

“…Cells were then illuminated with bright light, and red fluorescence was monitored over a time course of a few minutes. The transition to State 1 results in a fluorescence rise on a timescale of a few seconds to a few minutes (Schluchter et al, 1996;Aspinwall et al, 2004). The bright illumination ensures that the kinetics of the state transition are not limited by the kinetics of electron transport, and in the presence of DCMU, PSII centers remain closed, so that the fluorescence kinetics are not complicated by changes in PSII trap closure (Schluchter et al, 1996;Aspinwall et al, 2004).…”

“…Recent work has led to a consensus that state transitions involve changes in the relative energy transfer from phycobilisomes to the PSI and PSII reaction centers (Mullineaux, 1994;Schluchter et al, 1996;McConnell et al, 2002). This is usually accompanied by a redistribution of chlorophyll-absorbed energy, although the two effects can be separated (Emlyn-Jones et al, 1999;McConnell et al, 2002).…”

Lateral Segregation of Photosystem I in Cyanobacterial Thylakoids

“…A further subunit, PsaF, is located on the side of the PSI monomer opposite to PsaL; in green algae and plants, the lumenal face of this subunit is involved in binding plastocyanin and cytochrome c 6 (Farah et al, 1995), although not in T. elongatus (Mühlenhoff and Chauvat, 1996). The remaining subunits, PsaI, PsaJ, PsaK, PsaM, and PsaX, are small polypeptides believed to be involved with stabilizing PSI; loss of any one of these proteins does not significantly reduce the functionality of the protein complex (Schluchter et al, 1996;Xu et al, 1994;Naithani et al, 2000).…”