Stress Responses Regulated by SigR and WblC in Streptomyces coelicolor

Abstract

Cells modulate gene expressions to adapt environmental changes. Among the regulators that respond to environmental changes, SigR is one of the most abundant sigma factor in Streptomyces coelicolor and governs the thiol-oxidative stress response. RsrA, a redox-sensitive anti-sigma of SigR, senses redox changes through zinc coordinating histidine-cysteine residues and modulates SigR activity. In order to better understand the physiological function of SigR, chromatin immunoprecipitation combined with sequence and transcript analyses were conducted, revealing the 108 SigR target promoters. In addition to reported genes for thiol homeostasis, protein degradation and ribosome modulation, 64 additional operons were identified suggesting new functions of this global regulator. By analyzing the newly found target genes, it was demonstrated that SigR is required to maintain the level and activity of the housekeeping sigma factor HrdB during thiol-oxidative stress, and to protect cells against ultraviolet and thiol-reactive damages, by regulating UvrA. Next I found that antibiotics induce SigR and its regulon via a redox-independent pathway, leading to antibiotic resistance. The amplification of response to thiol-oxidative stress is achieved by producing an unstable isoform of SigR called SigR′ which is degraded in minutes. Unlike this transient response amplification mediated by unstable SigR', antibiotics induce stable SigR eliciting a prolonged response. Among the antibiotics, the sigRp1 transcripts is induced by sub-minimal inhibitory concentration of translation-inhibiting antibiotics, resulting in increased synthesis of the stable SigR. I also found that the increased expression of SigR by antibiotics was mediated by WblC/WhiB7, a DNA-binding protein similar to WhiB. WblC has three transcriptional start sites and wblC transcripts increased in all three promoters by antibiotic treatment. The amount of WblC protein and its binding to the sigRp1 promoter in vivo increased upon antibiotic treatment. By comparing antibiotic susceptibility of mutant and wild type, it was confirmed that SigR and WblC confer resistance to translation-inhibiting antibiotics. In addition, the sigR-homologous genes in Mycobacterium tuberculosis were induced by antibiotics as well. These findings reveal a novel antibiotic-induced resistance mechanism conserved among actinomycetes and give an example of overlap in cellular damage and defense mechanisms between thiol-oxidative and anti- translational stresses.