SUMMARY: We isolated four azide-resistant secA mutants of Bacillus subtilis and found that all of them were the result of a single amino acid replacement of threonine 128 of SecA by alanine or isoleucine. In the presence of 1·5 mM sodium azide, cell growth and protein translocation of the wild-type strain were completely inhibited, but those of the azide-resistant mutant strains were not. Wild-type and two mutant SecA proteins were purified. Both the basal level and the elevated ATPase activity of the mutant SecA proteins were threefold higher than those of the wild-type SecA. The elevated ATPase activity of the SecA mutants was reduced upon the addition of 1·5 mM sodium azide by only 5-10% as compared with 40% for that of the wild-type. These results indicate that the elevated ATPase activity of the SecA mutants is resistant to sodium azide and that it is also required for the protein translocation process of B. subtilis.


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