mrpA, a gene with roles in resistance to Na+ and adaptation to alkaline pH in the cyanobacterium Anabaena sp. PCC7120

Abstract

Transposon mutagenesis ofAnabaenasp. PCC7120 led to the isolation of a mutant strain, PHB11, which grew poorly at pH values above 10. The mutant strain exhibited pronounced Na+sensitivity; this sensitivity was higher under basic conditions. Mutant PHB11 also showed an inhibition of photosynthesis that was much more pronounced at alkaline pH. Reconstruction of the transposon mutation of PHB11 in the wild-type strain reproduced the phenotype of the original mutant. The wild-type version of the mutated gene was cloned and the mutation complemented. In mutant strain PHB11, the transposon had inserted within an ORF that is part of a seven-ORF operon with significant sequence similarity to a family of bacterial operons that are believed to code for a novel multiprotein cation/proton antiporter primarily involved in resistance to salt stress and adaptation to alkaline pH. TheAnabaenaoperon was denotedmrp(multiple resistance and pH adaptation) following the nomenclature of theBacillus subtilisoperon; the ORF mutated in PHB11 corresponded tomrpA. Computer analysis suggested that all seven predictedAnabaenaMrp proteins were highly hydrophobic with several transmembrane domains; in fact, the predicted protein sequences encoded bymrpA,mrpBandmrpCshowed significant similarity to hydrophobic subunits of the proton pumping NADH : ubiquinone oxidoreductase.In vivoexpression studies indicated thatmrpAis induced with increasing external Na+concentrations and alkaline pH;mrpAis also upregulated under inorganic carbon (Ci) limitation. The biological significance of a putative cyanobacterial Mrp complex is discussed.