Mutations in a putative chloride efflux transporter gene suppress the chloride requirement of photosystem II in the cytochrome c550-deficient mutant

The cytochrome c550-deficient mutant (psbV-disruptant) of Synechocystis requires a high concentration of Cl- in the culture medium to support photosynthetic oxygen evolution. From this disruptant, we isolated spontaneous suppressor mutants that are able to grow photoautotrophically in the absence of Cl-. Three independent mutations were identified: one was a deletion in slr0753 and two were a transposition of related insertion sequences in the same slr0753. The deduced product of slr0753 belongs to a novel group of the superfamily of ion efflux pumps and ion transporters. These results suggest that Slr0753 exports Cl- or a related anion, which is essential for PSII oxygen evolution.