Iron- and Quorum-sensing Signals Converge on Small Quorum-regulatory RNAs for Coordinated Regulation of Virulence Factors in Vibrio vulnificus.

Vibrio vulnificus is a marine bacterium that causes human infections resulting in high mortality. This pathogen harbors five quorum-regulatory RNAs (Qrr1-5) that affect the expression of pathogenicity genes by modulating the expression of the master regulator SmcR. The qrr genes are activated by phosphorylated LuxO to different degrees; qrr2 is strongly activated; qrr3 and qrr5 are moderately activated, and qrr1 and qrr4 are marginally activated and are the only two that do not respond to cell density-dependent regulation. Qrrs function redundantly to inhibit SmcR at low cell density and fully repress when all five are activated. In this study, we found that iron inhibits qrr expression in three distinct ways. First, the iron-ferric uptake regulator (Fur) complex directly binds to qrr promoter regions, inhibiting LuxO activation by competing with LuxO for cis-acting DNA elements. Second, qrr transcription is repressed by iron independently of Fur. Third, LuxO expression is repressed by iron independently of Fur. We also found that, under iron-limiting conditions, the five Qrrs functioned additively, not redundantly, to repress SmcR, suggesting that cells lacking iron enter a high cell density mode earlier and could thereby modulate expression of virulence factors sooner. This study suggests that iron and quorum sensing, along with their cognate regulatory circuits, are linked together in the coordinated expression of virulence factors.