SIGMA(R), AN RNA-POLYMERASE SIGMA-FACTOR THAT MODULATES EXPRESSION OFTHE THIOREDOXIN SYSTEM IN RESPONSE TO OXIDATIVE STRESS IN STREPTOMYCES-COELICOLOR A3(2)

We have identified an RNA polymerase sigma factor, sigma(R), that is p art of a system that senses and responds to thiol oxidation in the Gra m-positive, antibiotic-producing bacterium Streptomyces coelicolor A3( 2), Deletion of the gene (sigR) encoding sigma(R) caused sensitivity t o the thiol-specific oxidant diamide and to the redox cycling compound s menadione and plumbagin, This correlated with reduced levels of disu lfide reductase activity and an inability to induce this activity on e xposure to diamide, The trxBA operon, encoding thioredoxin reductase a nd thioredoxin, was found to be under the direct control of sigma(R). trxBA is transcribed from two promoters, trxBp1 and trxBp2, separated by 5-6 bp, trxBp1 is transiently induced at least 50-fold in response to diamide treatment in a sigR-dependent manner. Purified sigma(R) dir ected transcription from trxBp1 in vitro, indicating that trxBp1 is a target for sigma(R). Transcription of sigR itself initiates at two pro moters, sigRp1 and sigRp2, which are separated by 173 bp. The sigRp2 t ranscript was undetectable in a sigR-null mutant, and purified sigma(R ) could direct transcription from sigRp2 in vitro, indicating that sig R is positively autoregulated. transcription from sigRp2 was also tran siently induced (70-fold) following treatment with diamide. We propose a model in which sigma(R) induces expression of the thioredoxin syste m in response to cytoplasmic disulfide bond formation. Upon reestablis hment of normal thiol levels, sigma(R) activity is switched off, resul ting in down-regulation of trxBA and sigR. We present evidence that th e sigma(R) system also functions in the actinomycete pathogen Mycobact erium tuberculosis.