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

Abstract

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 ironlimiting 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.

Role of Non-coding Regulatory RNA in the Virulence of Human Pathogenic Vibrios

“…smcR mutants show attenuated cytotoxicity compared to the wild type (Lee et al, 2007). Wen et al (2016) showed that Qrr sRNAs in V. vulnificus are regulated via LuxO and modulate the expression of virulence factors via SmcR. Interestingly, the authors also showed that Qrr sRNAs are responsive to iron concentration, and that iron and QS converge on Qrrs to control the expression of virulence factors (Wen et al, 2016) ( Figure 2 ).…”

“…Small non-coding RNAs can modulate the expression of genes associated with survival and virulence by pairing with untranslated regions of genes in response to cell density, through the QS pathway (Wen et al, 2016). QS is a process of cell communication by which a bacterial population expresses a set of genes in a coordinated way in response to the cell density in the population (Hammer and Bassler, 2007; Shao and Bassler, 2012, 2014; Van Kessel et al, 2015) through the production, release, detection and response to extracellular signal molecules called autoinducers (Tu and Bassler, 2007; Rutherford et al, 2011).…”

“…Conversely, the four Qrr (Qrr1-4) of V. cholerae sRNAs are functionally redundant because any of its Qrr is sufficient to repress its master transcriptional regulator (Svenningsen et al, 2008). V. vulnificus has Qrr1-5 sRNAs that function additively, not redundantly, to repress SmcR (Wen et al, 2016). The only Qrr described in V. fischeri represses expression of the master regulator LitR (Kimbrough and Stabb, 2016) while in V. parahaemolyticus just three Qrr (Qrr2-4) sRNAs are known (Zhang et al, 2012).…”

“…Similarly, Qrr sRNAs affect the expression of pathogenicity genes in V. vulnificus by modulating the expression of the smcR ( Figure 2 ). The quorum-sensing pathway is similar to V. cholerae , and SmcR is responsible for regulating the expression of diverse virulence factors (Wen et al, 2016). smcR mutants show attenuated cytotoxicity compared to the wild type (Lee et al, 2007).…”

Quorum Sensing Gene Regulation by LuxR/HapR Master Regulators in Vibrios

“…The autoinducer-receptor systems in V. parahaemolyticus are likely similar to those in V. harveyi, based on phylogenetic analyses and signaling assays (10,14,16,42), whereas only the LuxPQ system that detects AI-2 has been identified in V. vulnificus (40). The number of Qrrs in different Vibrio species varies from one to five among vibrios, and they act either additively or redundantly (15,45,(52)(53)(54)(55)(56). Further, the Qrr regulon in V. harveyi (ϳ20 genes) does not appear to include the type VI secretion genes, which are regulated by the Qrrs in V. cholerae (50,57).…”