Structural and Functional Insights into Bacillus subtilis Sigma Factor Inhibitor, CsfB

Summary Global changes in bacterial gene expression can be orchestrated by the coordinated activation/deactivation of alternative sigma (σ) factor subunits of RNA polymerase. Sigma factors themselves are regulated in myriad ways, including via anti-sigma factors. Here, we have determined the solution structure of anti-sigma factor CsfB, responsible for inhibition of two alternative sigma factors, σ G and σ E , during spore formation by Bacillus subtilis . CsfB assembles into a symmetrical homodimer, with each monomer bound to a single Zn 2+ ion via a treble-clef zinc finger fold. Directed mutagenesis indicates that dimer formation is critical for CsfB-mediated inhibition of both σ G and σ E , and we have characterized these interactions in vitro . This work represents an advance in our understanding of how CsfB mediates inhibition of two alternative sigma factors to drive developmental gene expression in a bacterium.