Crystal structures of Streptomyces tsukubaensis sigma factor SigG1 and anti-sigma RsfG

Transcription of specific genes in bacteria under environmental stress is frequently initiated by extracytoplasmic function (ECF) a factors. ECFs a factors harbour two conserved domains, a2 and a4, for transcription initiation by recognition of the promoter region and recruitment of RNA polymerase (RNAP). The crystal structure of Strep-tomyces tsukubaensis SigG1, an ECF56-family a factor, was determined revealing a2, a4 and the additional carboxi-terminal domain SnoaL_2 tightly packed in a compact conformation. The structure of anti-sigma RsfG was also determined by X-ray crystallography and shows a rare B-barrel fold. Analysis of the metal binding motifs inside the protein barrel are consistent with Fe(III) binding, which is in agreement with previous findings that the Streptomyces tsukubaensis ECF56 SigG1-RsfG system is involved in metal-ion homeostasis.