THE BACILLUS-SUBTILIS SIGMA(X) PROTEIN IS AN EXTRACYTOPLASMIC FUNCTION SIGMA-FACTOR CONTRIBUTING TO SURVIVAL AT HIGH-TEMPERATURE

The sigX gene, identified as part of the international effort to seque nce the Bacillus subtilis genome, has been proposed to encode an alter native a factor of the extracytoplasmic function (ECF) subfamily, The sigX gene is cotranscribed with a downstream gene, ypuN, during logari thmic and early stationary phases of growth, We now report that strain s lacking sigma(X) are impaired in the ability to survive at high temp erature whereas a ypuN mutant has increased thermotolerance. We overpr oduced and purified sigma(X) from Escherichia coli and demonstrate tha t in vitro, both sigma(A) and sigma(X) holoenzymes recognize promoter elements within the sigX-ypuN control region, However, they have disti nct salt optima such that sigma(A)-dependent transcription predominate s at low salt while ax-dependent transcription predominates at high sa lt, A 54-bp region upstream of sigX suffices as a ox-dependent promote r in vivo, demonstrating that sigX is at least partially under positiv e autoregulatory control, Mutation of ypuN increases expression from t he ox-dependent promoter in vivo, suggesting that ypuN may encode a ne gative regulator of sigma(X) activity.