Cloning, sequencing and expression of the nhaA and nhaR genes from Salmonella enteritidis

Na+/H+ antiporter activity is wide-spread and plays essential physiological roles. We found that several Enterobacteriaceae share conserved sequences with nhaA, the gene coding for an E. coli antiporter. A delta nhaA strain, which is sensitive to Na+ and Li+, was used to clone by complementation a DNA fragment from Salmonella enteritidis which confers resistance to the ions. The cloned fragment increased Na+/H+ antiport activity in membranes isolated from strains carrying the respective hybrid plasmid. DNA sequence analysis of the insert revealed two open reading frames. Both encode putative polypeptides which are closely homologous to the nhaA and nhaR gene products from Escherichia coli. The antiporter activity displays properties very similar to that of the E. coli NhaA, namely, it is activated by alkaline pH and recognizes Li+ with high affinity.