5.3) Fis and RgsA in Pseudomonas
Following the hypothesis that several sRNAs regulate Fis, the first demonstration of sRNA regulation of Fis was conducted in 2016 in theP. aeruginosa PAO1 strain (Lu et al. , 2016). RgsA, originally named P16, is a 122 nt sRNA that is phylogenetically conserved in Pseudomonas species (Livny et al. , 2006). RgsA is regulated directly by RpoS and indirectly by the two-component system GacA/GacS (González et al. , 2008). Its expression increases from the exponential to the stationary phase, consistent with RpoS regulation, and decreases as cells are grown into deep stationary phase (González et al. , 2008). RgsA has also been found to be involved in oxidative stress response and swarming motility (Gonzálezet al. , 2008) (Park et al. , 2013). The predicted secondary structure of RgsA consists of four stem-loop structures. The third stem-loop is preceded by a highly conserved single-stranded region, which is a key determinant for base-pairing to fis mRNAs. In particular, two functional start codons were identified in thefis mRNA, the second of which is predominantly used in P. aeruginosa PAO1, resulting in the production of two types of Fis proteins. RgsA represses the expression of both proteins by different mechanisms. RgsA can repress the expression of the longer Fis by binding to the CDS region, leading to an acceleration of the RNase E-dependent mRNA decay. This interaction also inhibits the expression of shorter Fis by translational blockage and RNase cleavage. As with many sRNAs, Lu et al., (2016) demonstrated that RgsA regulates fis mRNA in an Hfq-dependent manner. More interestingly, they also showed that RgsA promoter activity is reduced in an hfq mutant, implying that Hfq also regulates RgsA at the transcriptional level. Furthermore, a feed-forward regulatory loop between Fis and RgsA was suggested after observing that the ectopic overexpression of RgsA was much higher in anrgsA mutant compared to the wild-type strain. Knowing that Fis represses rpoS transcription in Salmonella (Hirsch and Elliott, 2005), the authors proposed to investigate the regulatory effect of Fis on RpoS in PAO1. If so, this would suggest a regulatory loop involving RpoS as a third player.