Functional analysis of bacterial multi-subunit cation/H+ antiporter.

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The pha1 gene cluster (pha1A'-G) of Sinorhizobium meliloti had been characterized as a necessary component for proper invasion into plant root tissue. It has been suggested to encode a multi-subunit K+/H+ antiporter, since mutations in the pha1 region rendered S. meliloti cells K+ and alkaline sensitive. However, the detailed transport properties of the Pha1 system are yet to be determined. We have conducted the functional expression of the Pha1 system in Escherichia coli and the measurement of cation/H+ antiport activity, and showed that the Pha1 system is indeed a K+/H+ antiporter with a pH optimum at mildly alkaline conditions. Moreover, we found that the Pha1 system can transport Na+; this was unexpected based on previous phenotypical analyses of pha1 mutants. Furthermore, we demonstrated that the cation selectivity of the Pha1 system was altered when the pH was lowered from its optimum pH.