Thioredoxin, a small, ubiquitous protein which participates in redox reactions through the reversible oxidation of its active center dithiol to a disulfide, is an essential protein in Bacillus subtilis. A variety of stresses, including heat or salt stress or ethanol treatment, strongly enhanced the synthesis of thioredoxin in B. subtilis. The stress induction of the monocistronic trxA gene encoding thioredoxin occurs at two promoters. The general stress sigma factor, sigmaB, was required for the initiation of transcription at the upstream site, S(B), and the promoter preceding the downstream start site, S(A), was presumably recognized by the vegetative sigma factor, sigmaA. In contrast to the heat-inducible, sigmaA-dependent promoters preceding the chaperone-encoding operons groESL and dnaK, no CIRCE (for controlling inverted repeat of chaperone expression) was present in the vicinity of the start site, S(A). The induction patterns of the promoters differed, with the upstream promoter displaying the typical stress induction of sigmaB-dependent promoters. Transcription initiating at S(A), but not at S(B), was also induced after treatment with hydrogen peroxide or puromycin. Such a double control of stress induction at two different promoters seems to be typical of a subgroup of class III heat shock genes of B. subtilis, like clpC, and it either allows the cells to raise the level of the antioxidant thioredoxin after oxidative stress or allows stressed cells to accumulate thioredoxin. These increased levels of thioredoxin might help stressed B. subtilis cells to maintain the native and reduced state of cellular proteins.
