The ftsH gene of Bacillus subtilis is transiently induced after osmotic and temperature upshift

Abstract

The ftsH gene of Bacillus subtilis has been identified as a salt-sensitive insertion mutation in strain UG1. Here, we show that UG1 has an insertion near the 3 end of ftsH. The salt sensitivity of this mutant was caused by reduction of ftsH mRNA levels by the synthesis of an artificial antisense RNA originating at a promoter located within the insertion and reading backwards into the ftsH gene. The salt-sensitive phenotype could be overcome by deleting the promoter from which the antisense RNA was transcribed. A physiological analysis of the isogenic wild-type strain in minimal medium revealed unimpaired growth at up to 1 M NaCl, and growth above 1.2 M NaCl was observed only after addition of the osmoprotectant proline or glycine betaine. In contrast, growth of strain UG1 was reduced at a salt concentration above 0.2 M, which could be rescued by the two compatible solutes already mentioned and also by trehalose. Primer extension revealed one potential transcription start site downstream of a putative vegetative promoter, which was activated after osmotic or temperature upshift. Northern (RNA blot) experiments led to the detection of a 2.1-kb transcript, suggesting that ftsH is monocistronic. A transcriptional fusion between ftsH and the gus reporter gene exhibited a twofold increase in b-glucuronidase activity after osmotic upshift. To further confirm the need for an enhanced level of FtsH protein after osmotic upshift, the promoter was replaced by the sucrose-inducible promoter PsacB. Whereas this mutant strain could grow in the absence of inducer in LB medium, it stopped growth immediately after addition of 1.1 M NaCl. We conclude that an increased amount of FtsH protein is essential for B. subtilis to cope with an increase in osmolarity or temperature.