The gene sll0273 of the cyanobacterium Synechocystis sp. strain PCC6803 encodes a protein essential for growth at low Na+/K+ ratios
A mutant of Synechocystis sp. strain PCC6803 was obtained by random cartridge mutagenesis, which could not grow at low sodium concentrations. Genetic analyses revealed that partial deletion of the sll0273 gene, encoding a putative Na + /H + exchanger, was responsible for this defect. Physiological characterization indicated that the sll0273 mutant exhibited an increased sensitivity towards K + , even at low concentrations, which was compensated for by enhanced concentrations of Na + . This enhanced Na + demand could also be met by Li + . Furthermore, addition of monensin, an ionophore mediating electroneutral Na + /H + exchange, supported growth of the mutant at unfavourable Na + /K + ratios. Measurement of internal Na + and K + contents of wild-type and mutant cells revealed a decreased Na + /K + ratio in mutant cells pre-incubated at a low external Na + /K + ratio, while it remained at the level of the wild type after pre-incubation at a high external Na + /K + ratio. We conclude that the Sll0273 protein is required for Na + influx, especially at low external Na + concentrations or low Na + /K + ratios. This system may be part of a sodium cycle and may permit re-entry of Na + into the cells, if nutrient/Na + symporters are not functional or operating.