The Bacterial Transcription Termination Factor Rho Coordinates Mg2 + Homeostasis with Translational Signals

• The widespread Mg2+ channel CorA was thought to be constitutively expressed.
• Salmonella corA leader mRNA harbors a Rho-dependent transcription terminator.
• The corA leader can adopt two conformations that control Rho access to the mRNA.
• Translation of a short open reading frame in the corA leader dictates the mRNA conformation.
• Rho links expression of all Salmonella Mg2 + transporters to translational signals.

The bacterial protein Rho triggers transcription termination at the ends of many operons and when transcription and translation become uncoupled. In addition to these genome wide activities, Rho implements regulation of specific genes by dictating whether RNA polymerase terminates transcription within the 5′ leader region or continues into the downstream coding region. Here, we report that the Mg2 + channel gene corA in Salmonella enterica serovar Typhimurium, which was previously thought to be constitutively expressed, is regulated by a Rho-dependent terminator located within its 5′ leader region. We demonstrate that the unusually long and highly conserved corA leader mRNA can adopt two mutually exclusive conformations that determine whether or not Rho interacts with a Rho utilization site on the nascent RNA and thereby prevents transcription of the corA coding region. The RNA conformation that promotes Rho-dependent termination is favored by efficient translation of corL, a short open reading frame located within the corA leader. Thus, corA transcription is inversely coupled to corL translation. This mechanism resembles those governing expression of Salmonella's other two Mg2 + transport genes, suggesting that Rho links Mg2 + uptake to translational signals.

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