Osmotic control of proU transcription is mediated through direct action of potassium glutamate on the transcription complex.

Osmoregulated transcription from the proU promoter of Escherichia coli has been successfully reconstituted from purified components in a simple in vitro system consisting of plasmid DNA template, RNA polymerase, and nucleotides in the absence of any other protein factor. proU transcription is stimulated by addition of high concentrations of potassium glutamate, the ionic compound accumulated in vivo during hyperosmotic stress. Transcription from the nonosmoregulated promoters beta la, lac, and pepN is inhibited under the same conditions, demonstrating the specificity of potassium glutamate as an inducer of proU transcription. proU transcription requires a circular DNA template, but stable alterations in the degree of supercoiling are unnecessary for this potassium glutamate-dependent signaling. These results agree well with previous data obtained in an S-30 coupled transcription/translation system and suggest that physiological changes in the ionic composition of the intracellular millieu can regulate gene expression.