Abstract: Alternate sigma factors provide an effective way of diversifying bacterial gene expression in response to environmental changes. In Streptomyces coelicolor where more than 65 sigma factors are predicted, sigma(R) is the major regulator for response to thiol-oxidative stresses. sigma(R) becomes available when its bound anti-sigma factor RsrA is oxidized at sensitive cysteine thiols to form disulphide bonds. sigma(R) regulon includes genes for itself and multiple thiol-reducing systems, which constitute positive and negative feedback loops respectively. We found that the positive amplification loop involves an isoform of sigma(R) (sigma(R')) with an N-terminal extension of 55 amino acids, produced from an upstream start codon. A major difference between constitutive sigma(R) and inducible sigma(R') is that the latter is markedly unstable (t(1/2) approximately 10 min) compared with the former (> 70 min). The rapid turnover of sigma(R') is partly due to induced ClpP1/P2 proteases from the sigma(R) regulon. This represents a novel way of elaborating positive and negative feedback loops in a control circuit. Similar phenomenon may occur in other actinomycetes that harbour multiple start codons in the sigR homologous gene. We observed that sigH gene, the sigR orthologue in Mycobacterium smegmatis, produces an unstable larger isoform of sigma(H) upon induction by thiol-oxidative stress.