Structure of a nonheme globin in environmental stress signaling.

RsbR is a regulator of sigma(B), the RNA polymerase sigma factor subunit responsible for transcribing the general stress response genes when environmental stress is imposed on Bacillus subtilis. The C-terminal domain of RsbR and its paralogues is a substrate for the kinase function of another sigma(B) regulator, RsbT, but the amino acid sequence of the N-terminal domain of RsbR does not reveal any obvious biochemical function. RsbR, its paralogues, and other regulators of sigma(B), including RsbS and RsbT, form large signaling complexes, called stressosomes. We have determined and present here the crystal structure of the N-terminal domain of RsbR. Unexpectedly, this structure belongs to the globin fold superfamily, but there is no bound cofactor. The globin domain from globin-coupled sensory systems replaces the N-terminal domain of RsbR in some bacteria, indicating a common genetic ancestry for RsbR and the globin family. We suggest that the globin fold has been "recycled" in RsbR and that one more activity can be included in the repertoire of globin functions, namely the ability to bind signaling macromolecules such as RsbT.