Analysis of mutations to suppress the chloride requirement of the psbV-disruptant of the cyanobacterium Synechocystis sp. PCC 6803

Abstract

Chloride ion (Cl-) is known as an essential cofactor for the water-splitting reaction of PSII. Cyanobacterial mutants defect in the extrinsic proteins of PSII such as cytochrome c550 (encoded by psbV) require high concentration of Cl- in growth medium for photoautotrophic growth. From the psbV-disruptant, we isolated suppressor mutants, which recovered the growth in the low Cl- medium. Genomic DNA library was prepared from the mutants and screened by complementation of the growth defect of the psbV-disruptant. Three independent mutations were identified. One was a deletion harboring the initiation codon region of slr0753. The other two had transposition of insertion sequences (ISY203) within slr0753 at different loci. These suggest that inactivation of slr0753 strictly coupled with escape from the Cl--requirement of the psbV-disruptant. The deduced product of slr0753 is highly homologous to conserved ORFs in prokaryotes, which putatively encode some transportors. It is also homologous to the arsenic efflux pump protein of E. coli. These results suggest that the slr0753 product is an efflux pump of intracellular Cl- or some anions, which are critical for water oxidation in the mutant PSII.