Application of protoplast transformation and single- and double-crosso
ver mutagenesis protocols to alkaliphilic Bacillus firmus OF4811M (an
auxotrophic strain of B. firmus OF4) facilitated the extension of the
sequence of the previously cloned nhaC gene, which encodes an Na+/H+ a
ntiporter, and the surrounding region, The nhaC gene is part of a like
ly 2-gene operon encompassing nhaC and a small gene that was designate
d nhaS; the operon is preceded by novel direct repeats, The predicted
alkaliphile NhaC, based on the extended sequence analysis, would be a
membrane protein with 462 amino acid residues and 12 transmembrane seg
ments that is highly homologous to the deduced products of homologous
genes of unknown function from Bacillus subtilis and Haemophilus influ
enzae. The full-length version of nhaC complemented the Na+-sensitive
phenotype of an antiporter-deficient mutant strain of Escherichia coli
but not the alkali-sensitive growth phenotypes of Na+/H+-deficient mu
tants of either alkaliphilic B. firmus OF4811M or B. subtilis. Indeed,
NhaC has no required role in alkaliphily, inasmuch as the nhaC deleti
on strain of B. firmus OF4811M, N13, grew well at pH 10.5 at Na+ conce
ntrations equal to or greater than 10 mM. Even at lower Na+ concentrat
ions, N13 exhibited only a modest growth defect at pH 10.5, This was a
ccompanied by a reduced capacity to acidify the cytoplasm relative to
the medium compared to the wild-type strain or to N13 complemented by
cloned nhaC. The most notable deficiency observed in N13 was its poor
growth at pH 7.5 and Na+ concentrations up to 25 mM. During growth at
pH 7.5, NhaC is apparently a major component of the relatively high af
finity Na+/H+ antiport activity available to extrude the Na+ and to co
nfer some initial protection in the face of a sudden upshift in extern
al pH, i.e., before full induction of additional antiporters. Consiste
nt with the inference that NhaC is a relatively high affinity, electro
genic Na+/H+ antiporter, N13 exhibited a defect in diffusion potential
-energized efflux of Na-22(+) from right-side-out membrane resides fro
m cells that were preloaded with 2 mM Na+ and energized at pH 7.5. Whe
n the experiment was conducted with vesicles loaded with 25 mM Na+, co
mparable efflux was observed in preparations from all the strains.