Deletion of ant in Escherichia coli reveals its function in adaptation to high salinity and an alternative Na⁺/H⁺ antiporter system(s)

We have deleted the chromosomal ant gene from Escherichia coli by substitution with the kan gene, which encodes kanamycin resistance. The Δant strains obtained cannot adapt to high sodium concentrations (700 mM, pH 6.8), which do not affect the wild type. The Na⁺ sensitivity of Δant is pH dependent, increasing at alkaline pH. Thus at pH 8.5, 100 mM NaCl retard growth of Δant with no effect on the wild type. The Δant strains also cannot challenge the toxic effects of Li⁺ ions, a substrate of the Na⁺/H⁺ antiporter system. However, growth of these strains is normal on carbon sources which require Na⁺ ions for transport and growth. Moreover, antiporter activity, as measured in everted membrane vesicles, is not significantly impaired. A detailed analysis of the remaining antiporter activity in a Δant strain reveals kinetic properties which differ from those displayed by the ant protein: (a) K(m) for transport of Li⁺ ions is about 15 times higher and (b) the activity is practically independent of intracellular pH. Our results demonstrate the presence of an alternative Na⁺/H⁺ antiporter(s) in E. coli, additional to ant system.