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

A mutant, Δ, was generated to functionally characterize Sll1252. Δ exhibited a slow-growth phenotype at 70 µmol photons m s and glucose sensitivity. In Δ, the rate of PSII activity was not affected, whereas the whole chain electron transport activity was reduced by 45%. The inactivation of 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 . The inhibitory effect of DCMU on PSII activity was similar in both wild-type and Δ. However, the concentration of DBMIB for 50% inhibition of whole chain electron transport activity was 140 nM for Δ and 300 nM for wild-type, confirming the site of action of Sll1252. Moreover, the elevated level of the reduced-PQ pool in Δ supports that Sll1252 functions between the PQ pool and Cyt . Interestingly, we noticed that Δ reverted to wild-type phenotype by insertion of natural transposon, ISY523, at the disruption site. Δ, expressing only the C-terminal region of Sll1252, exhibited a slow-growth phenotype and disorganized thylakoid structure compared to wild-type and Δ (expressing only the N-terminal region). Collectively, our data suggest that Sll1252 regulates electron transfer between the PQ pool and the Cyt complex in the linear photosynthetic electron transport chain via coordinated function of both the N- and C-terminal regions of Sll1252.