Disulphide stress and regulation
Disulfide stress and its regulation by σR-RsrA complex
The sigma factor σRand its inhibitor RsrA regulate the expression of ~30 genes in response to disulfide stress in the soil bacterium Streptomyces coelicolor. RsrA is a zinc-containing anti-sigma (ZAS) protein, one of a growing list of transcriptional regulators that respond to diverse environmental cues. During oxidative (disulfide) stress, a trigger disulfide forms in RsrA that causes the expulsion of its single metal ion. This results in a large structural change that abolishes its ability to bind σR, which is then free to activate transcription of its target genes, including thioredoxin and thioredoxin reductase. Redox homeostasis is resumed when these cellular reductants re-reduce RsrA via the small molecule reducing agent myothiol. We found that the trigger disulfide is naturally degenerate, formed between all three of the metal ligands, Cys11, Cys41 and Cys44, which causes substantial changes in protein structure, as deduced by circular dichroism spectroscopy. See Kang et al EMBO J. (1999) 18, 4292, Li et al (2002) J. Mol. Biol. 323, 225 and Li et al (2003) J. Mol. Biol. 333, 461 for further details.
Defining the metal coordination of RsrA
Controversy surrounded the metal ligation chemistry of the single zinc ion in RsrA. We used a combination of mutagenesis and EXAFS spectroscopy to define this coordination chemistry, which includes the three cysteines known to be involved in forming the degenerate trigger disulfide bond in RsrA. Three of the ligands to the metal ion (His37xxxCys41xxCys44) are part of a sequence motif which is invariant in ZAS proteins. See Zdanowski et al (2006) Biochemistry 45, 8294 for further details.