A conserved structural module regulates transcriptional responses to diverse stress signals in bacteria.

Summary A transcriptional response to singlet oxygen in Rhodobacter sphaeroides is controlled by the group IV σ factor σ E and its cognate anti-σ ChrR. Crystal structures of the σ E /ChrR complex reveal a modular, two-domain architecture for ChrR. The ChrR N-terminal anti-σ domain (ASD) binds a Zn 2+ ion, contacts σ E , and is sufficient to inhibit σ E -dependent transcription. The ChrR C-terminal domain adopts a cupin fold, can coordinate an additional Zn 2+ , and is required for the transcriptional response to singlet oxygen. Structure-based sequence analyses predict that the ASD defines a common structural fold among predicted group IV anti-σs. These ASDs are fused to diverse C-terminal domains that are likely involved in responding to specific environmental signals that control the activity of their cognate σ factor.