nhaG Na⁺/H⁺ antiporter gene of bacillus subtilis ATCC9372, which is missing in the complete genome sequence of strain 168, and properties of the antiporter

We cloned a gene which enabled Escherichia coli mutant host cells lacking all of the major Na⁺/H⁺ antiporters to grow in the presence of 0.2 M NaCl from chromosomal DNA of Bacillus subtilis ATCC9372. An Na⁺/H⁺ antiport activity was observed with membrane vesicles prepared from E. coli cells possessing the cloned gene, but not with vesicles from the host cells. Lithium ion was also a substrate for the antiporter. We sequenced the cloned DNA and found one open reading frame (designated nhaG) preceded by a promoter-like sequence and a Shine-Dalgarno sequence, and followed by a terminator-like sequence. The deduced amino acid sequence of NhaG suggested that it consisted of 524 residues and that the calculated molecular mass was 58.1 kDa. None of the bacterial Na⁺/H⁺ antiporters so far reported, except NhaP of Pseudomonas aeruginosa and SynNhaP (NhaS1) of Synechocystis sp., showed significant sequence similarity with the NhaG. However, the NhaP, the SynNhaP, animal NHEs (Na⁺/H⁺ exchangers), and some hypothetical N⁺/H⁺ antiporters of several organisms showed significant sequence similarities with the NhaG. Interestingly, the entire DNA region corresponding to the nhaG gene is missing in the reported complete genome sequence of B. subtilis strain 168. We detected a band that hybridized with the nhaG DNA in chromosomal DNA from B. subtilis ATCC9372 but not with that from strain 168. The missing DNA region (1,774 base pairs) is sandwiched by two identical sequences, TTTTCTT.