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

[Display omitted] •Streptomyces tsukubaensis sigma factor SigG1 adopts a compact conformation with σ2, σ4 and the SnoaL_2 domain tightly packed.•Anti-sigma RsfG shows a rare β-barrel fold.•RsfG has a putative Fe(III) binding site that may be associated with the role of the SigG1-RsfG system in metal homeostasis. Transcription of specific genes in bacteria under environmental stress is frequently initiated by extracytoplasmic function (ECF) σ factors. ECFs σ factors harbour two conserved domains, σ2 and σ4, for transcription initiation by recognition of the promoter region and recruitment of RNA polymerase (RNAP). The crystal structure of Streptomyces tsukubaensis SigG1, an ECF56-family σ factor, was determined revealing σ2, σ4 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 β-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.