Abstract
The pha1 gene cluster (pha1AB-G) of Sinorhizobium meliloti had been characterized as a necessary component for proper invasion into plant root tissue, and suggested to encode a multi-subunit K+/H+ antipoter (CPA-3) since mutations in pha1 region rendered S. meliloti cells K+ and alkaline sensitive. However, detailed property of Pha1 activity is yet to be determined. Interestingly, most of the CPA-3 transporters are highly selective to Na+, unlike Pha1. Here, we report the functional expression of Pha1 in E.coli TO114, which lacks three major cation/H+ antiporters, and the measurement of cation/H+ antiport activity. We show that the Pha1 system is a K+(Na+)/H+ antiporter. Furthermore, we found that the cation selectivity of Pha1 system is regulated by pH, which might reflect the importance of Pha1 system in S. meliloti to cope with different environmental conditions such as outside or inside of plant cells.
