Conformational toggle triggers a modulator of RNA polymerase activity.

Members of a recently discovered class of transcription factor, which includes the Gre factors that stimulate transcript cleavage, function by directly modulating the catalytic properties of RNA polymerase (RNAP). Now, three research groups have determined crystal structures of a Gre homolog, Gfh1, which inhibits all RNAP catalytic activities. Strikingly, these structures reveal a puzzling discrepancy between the Gfh1 and GreA conformations, but the discovery that a pH-dependent conformational toggle alters Gfh1 activity suggests an elegant solution.