Mutations that confer resistance to sodium azide affect the levels of two physiological isoforms of SecA in Escherichia coli

Sodium azide inhibits bacterial growth by inhibiting the SecA-dependent translocation of proteins across the cytoplasmic membrane, and all of the mutations known to confer increased resistance to azide are in the secA gene. However, the molecular mechanism of resistance is unclear. To gain insight into this mechanism, we designed a genetic screen to isolate transposon insertion mutations that conferred increased resistance to sodium azide, which were also unlinked to secA. We isolated six mutants, which we dubbed uar mutants, containing insertions in the rpsA, rpsQ, srmB and nusB genes. Growth of E. coli in the presence of azide causes increased accumulation of the larger of two previously noted, but uncharacterised, isoforms of SecA, but the uar mutations caused increased production of the smaller isoform in the presence of azide. Mutations in the secA gene that confer azide resistance also caused increased production of the small SecA isoform in the presence of azide, suggesting a shared mechanism. Expression of truncated variants of SecA, which lack the C-terminal tail, caused moderately increased resistance to azide, and western blotting analysis confirmed that the smaller SecA isoform lack the C-terminal ~70 amino acids. These results indicate that E. coli produces two SecA isoforms under normal growth conditions and that changes to the proportions of the isoform correlate with resistance to sodium azide.