Positive and negative feedback regulatory loops of thiol-oxidative stress response mediated by an unstable isoform of σ^R in actinomycetes

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, σ^R is the major regulator for response to thiol-oxidative stresses. σ^R becomes available when its bound antisigma factor RsrA is oxidized at sensitive cysteine thiols to form disulphide bonds. σ^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 σ^R (σ^R') with an N-terminal extension of 55 amino acids, produced from an upstream start codon. A major difference between constitutive σ^R and inducible σ^R' is that the latter is markedly unstable (t_1/2 ∼ 10 min) compared with the former (> 70 min). The rapid turnover of σ^R' is partly due to induced ClpP1/P2 proteases from the σ^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 σ^H upon induction by thiol-oxidative stress.