Sll1252 Coordinates Electron Transport between Plastoquinone and Cytochrome b Complex in Synechocystis PCC 6803

A mutant, Δsll1252, was generated to functionally characterize Sll1252. Δsll1252 exhibited a slow-growth phenotype at 70 µmol photons m^-2 s^-1 and glucose sensitivity. In Δsll1252, 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. The inhibitory effect of DCMU on PSII activity was similar in both wild-type and Δsll1252. However, the concentration of DBMIB for 50% inhibition of whole chain electron transport activity was 140 nM for Δsll1252 and 300 nM for wild-type, confirming the site of action of Sll1252. Moreover, the elevated level of the reduced-PQ pool in Δsll1252 supports that Sll1252 functions between the PQ pool and Cyt b. Interestingly, we noticed that Δsll1252 reverted to wild-type phenotype by insertion of natural transposon, ISY523, at the disruption site. Δsll1252-N, expressing only the C-terminal region of Sll1252, exhibited a slow-growth phenotype and disorganized thylakoid structure compared to wild-type and Δsll1252-C (expressing only the N-terminal region). Collectively, our data suggest that Sll1252 regulates electron transfer between the PQ pool and the Cyt b complex in the linear photosynthetic electron transport chain via coordinated function of both the N- and C-terminal regions of Sll1252.