Sll1252 Coordinates Electron Transport between Plastoquinone and Cytochrome b6/f Complex in Synechocystis PCC 6803

A mutant, Delta sll1252(ins), was generated to functionally characterize Sll1252. Delta sll1252(ins) exhibited a slow-growth phenotype at 70 mu mol photons m(-2) s(-1) and glucose sensitivity. In Delta sll1252(ins), the rate of PSII activity was not affected, whereas the whole chain electron transport activity was reduced by 45%. The inactivation of sll1252 led to the upregulation of genes, which were earlier reported to be induced in DBMIB-treated wild-type, suggesting that Sll1252 may be involved in electron transfer from the reduced-PQ pool to Cyt b(6)/f. The inhibitory effect of DCMU on PSII activity was similar in both wild-type and Delta sll1252(ins). However, the concentration of DBMIB for 50% inhibition of whole chain electron transport activity was 140 nM for Delta sll1252(ins) and 300 nM for wild-type, confirming the site of action of Sll1252. Moreover, the elevated level of the reduced-PQ pool in Delta sll1252(ins) supports that Sll1252 functions between the PQ pool and Cyt b(6)/f. Interestingly, we noticed that Delta sll1252(ins) reverted to wild-type phenotype by insertion of natural transposon, ISY523, at the disruption site. Delta sll1252-N-trn, expressing only the C-terminal region of Sll1252, exhibited a slow-growth phenotype and disorganized thylakoid structure compared to wild-type and Delta sll1252-C-trn (expressing only the N-terminal region). Collectively, our data suggest that Sll1252 regulates electron transfer between the PQ pool and the Cyt b(6)/f complex in the linear photosynthetic electron transport chain via coordinated function of both the N- and C-terminal regions of Sll1252.