Environmental regulation operating at the promoter clearance step of bacterial transcription

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Abstract
In vivo transcription of the Escherichia coli argO gene, which encodes an arginine (Arg) exporter, requires the LysR-family regulator protein ArgP (previously called IciA) and is induced in the presence of Arg or its naturally occurring antimetabolite analog canavanine. Lysine (Lys) addition, on the other hand, phenocopies an argP mutation to result in the shutoff of argO expression. We now report that the ArgP dimer by itself is able to bind the argO promoter-operator region to form a binary complex, but that the formation of a ternary complex with RNA polymerase is greatly stimulated only in presence of a coeffector. Both Arg and Lys were proficient as coeffectors for ArgP-mediated recruitment of RNA polymerase to, and open complex formation at, the argO promoter, although only Arg (but not Lys) was competent to activate transcription. The two coeffectors competed for binding to ArgP, and the ternary complex that had been assembled on the argO template in the presence of Lys could be chased into a transcriptionally active state upon Arg addition. Our results support a novel mechanism of argO regulation in which Lys-bound ArgP reversibly restrains RNA polymerase at the promoter, at a step (following open complex formation) that precedes, and is common to, both abortive and productive transcription. This represents, therefore, the first example of an environmental signal regulating the final step of promoter clearance by RNA polymerase in bacterial transcription. We propose that, in E. coli cells, the ternary complex remains assembled and poised at the argO promoter at all times to respond, positively or negatively, to instantaneous changes in the ratio of intracellular Arg to Lys concentrations.