Remodeling and activation of Escherichia coli RNA polymerase by osmolytes

The ability of bacteria to survive environmental stresses and colonize the gastrointestinal tract depends on adaptation to high osmolality. The adaptation involves global reprogramming of gene expression, including inhibition of bulk σ70 RNA polymerase transcription and activation of bulk σ38 transcription. The activating signal transduction pathways that originate with osmolytes remain to be established. Experiments here confirm that accumulation of a simple signaling molecule, glutamate, can reprogram RNA polymerase in vitro without the need for specific protein receptors. During osmotic activation, glutamate appears to act as a Hofmeister series osmolyte to facilitate promoter escape. Escape is accompanied by a remodeling of the key interaction between the σ38 stress protein and the β-flap of the bacterial core RNA polymerase. This activation event contrasts with the established mechanism of inhibition in which glutamate, by virtue of its electrostatic properties, helps to inhibit binding to ribosomal promoters after osmotic shock. Overall, Escherichia coli survival in natural hosts and reservoirs is expected to rely on the accumulation of simple ions that trigger changes in protein conformation that lead to global changes in transcription.