Linking the second messenger cyclic di-GMP signaling with the regulatory activity of the small RNA ErsA in Pseudomonas aeruginosa.

Biofilm formation in Pseudomonas aeruginosa is regulated by intersecting signals from the envelope stress response and the cyclic di-GMP (c-di-GMP) second messenger network. We previously described that the envelope stress response sigma factor σ22 can simultaneously drive transcriptional activation and, via the small RNA ErsA, post-transcriptional repression of algC, which encodes a central enzyme in the production of sugar precursors for biofilm matrix exopolysaccharides. The inhibitory effect of ErsA on AlgC appeared incongruent with the well-established role of high c-di-GMP levels in promoting exopolysaccharide synthesis. In this study, we address this apparent contradiction by demonstrating that elevated c-di-GMP levels relieve ErsA-mediated repression of AlgC. This regulatory mechanism also applies to other ErsA targets, including the envelope homeostasis lipoprotein MlaA and the flagellar regulator FleN, which cooperates with FleQ to promote the biosynthesis of Pel/Psl exopolysaccharides and the CdrAB adhesins under high c-di-GMP conditions. Collectively, our results support a model in which ErsA serves as a central, c-di-GMP-responsive regulatory hub, linking envelope stress responses to c-di-GMP cascades to fine-tune key extracytoplasmic functions and coordinate biofilm formation.