In Pseudomonas aeruginosa, the small RNA-binding, regulatory protein RsmA is a negative control element in the formation of several extracellular products (e.g., pyocyanin, hydrogen cyanide, PA-IL lectin) as well as in the production of N-acylhomoserine lactone quorum-sensing signal molecules. RsmA was found to control positively the ability to swarm and to produce extracellular rhamnolipids and lipase, i.e., functions contributing to niche colonization by P. aeruginosa. An rsmA null mutant was entirely devoid of swarming but produced detectable amounts of rhamnolipids, suggesting that factors in addition to rhamnolipids influence the swarming ability of P. aeruginosa. A small regulatory RNA, rsmZ, which antagonized the effects of RsmA, was identified in P. aeruginosa. Expression of the rsmZ gene was dependent on both the global regulator GacA and RsmA, increased with cell density, and was subject to negative autoregulation. Overexpression of rsmZ and a null mutation in rsmA resulted in quantitatively similar, negative or positive effects on target genes, in agreement with a model that postulates titration of RsmA protein by RsmZ RNA.Pseudomonas aeruginosa is a ubiquitous saprophyte and an opportunistic human pathogen which synthesizes numerous extracellular products including elastase, LasA protease, alkaline protease, phospholipase C, lipase, exotoxin A, rhamnolipids, hydrogen cyanide (HCN), and pyocyanin (25, 63). The production of these exoproducts, most of which can act as virulence factors, is positively controlled by two quorum-sensing signal molecules, N-(3-oxododecanoyl)-homoserine lactone (3-oxo-C12-HSL) and N-butanoyl-homoserine lactone (C4-HSL), which activate the transcription factors LasR and RhlR, respectively (20,26,59). The las and rhl systems are organized in a hierarchical manner such that the las system exerts transcriptional control over both rhlR and rhlI (26). A third signal molecule, 2-heptyl-3-hydroxy-4(1H)-quinolone, the synthesis and activity of which is linked to the las and rhl circuitry, is also required for virulence factor production and, in particular, for rhl-dependent exoproducts including pyocyanin and PA-IL lectin (41).Motility helps P. aeruginosa to colonize niches (10); three types of motility are observed, i.e., swimming, twitching, and swarming (24, 44). Whereas swimming in liquid media depends on flagella, twitching on solid media requires type IV pili.Swarming on semisolid media results from a combination of both types of motility and also requires rhamnolipid production (24, 46). Rhamnolipids are biosurfactants which not only enhance bacterial surface translocation by virtue of their wetting properties but also stimulate solubilization and degradation of hydrocarbons (35) and act as heat-stable hemolysins (21).The production of N-acyl-homoserine lactones (AHLs) and the expression of many virulence determinants in P. aeruginosa is negatively controlled at a posttranscriptional level by the small RNA-binding protein RsmA (42,43). This regulator is a homolog of CsrA in E...

Posttranscriptional control is known to contribute to the regulation of secondary metabolism and virulence determinants in certain gram-negative bacteria. Here we report the isolation of a Pseudomonas aeruginosa gene which encodes a global translational regulatory protein, RsmA (regulator of secondary metabolites). Overexpression of rsmA resulted in a substantial reduction in the levels of extracellular products, including protease, elastase, and staphylolytic (LasA protease) activity as well as the PA-IL lectin, hydrogen cyanide (HCN), and the phenazine pigment pyocyanin. While inactivation of rsmA in P. aeruginosa had only minor effects on the extracellular enzymes and the PA-IL lectin, the production of HCN and pyocyanin was enhanced during the exponential phase. The influence of RsmA on N-acylhomoserine lactone-mediated quorum sensing was determined by assaying the levels of N-(3-oxododecanoyl)homoserine lactone (3-oxo-C12-HSL) and N-butanoylhomoserine lactone (C4-HSL) produced by the rsmA mutant and the rsmA-overexpressing strain. RsmA exerted a negative effect on the synthesis of both 3-oxo-C12-HSL and C4-HSL, which was confirmed by using lasI and rhlI translational fusions. These data also highlighted the temporal expression control of the lasI gene, which was induced much earlier and to a higher level during the exponential growth phase in an rsmA mutant. To investigate whether RsmA modulates HCN production solely via quorum-sensing control, hcn translational fusions were employed to monitor the regulation of the cyanide biosynthesis genes (hcnABC). RsmA was shown to exert an additional negative effect on cyanogenesis posttranscriptionally by acting on a region surrounding the hcnA ribosome-binding site. This suggests that, in P. aeruginosa, RsmA functions as a pleiotropic posttranscriptional regulator of secondary metabolites directly and also indirectly by modulating the quorum-sensing circuitry.