Abstract

Significance MerR-family regulators act on suboptimal promoters to control the transcriptions of genes that help bacteria defend against a diverse set of metals and drugs. How they modulate RNA polymerase (RNAP) activity to control transcription initiation remains unclear, however. Here we show that CueR—a Cu + -responsive MerR-family metalloregulator—biases the kinetic sampling of RNAP binding events that lead to two noninterconverting states: a dead-end complex to repress or an open complex to activate transcription, constituting a branched pathway distinct from the linear pathway prevalent for transcription initiation at optimal promoters. This mechanistic insight contributes new fundamental knowledge to bacterial transcription regulation, and may help develop antibiotics that target this regulation mechanism to compromise bacterial defenses.