Dual function of the McaS small RNA in controlling biofilm formation

Many bacterial small RNAs (sRNAs) regulate gene expression through base-pairing with mRNAs, and it has been assumed that these sRNAs act solely by this one mechanism. Here we report that the multicellular adhesive (McaS) sRNA of Escherichia coli uniquely acts by two different mechanisms: base-pairing and protein titration. Previous work established that McaS base pairs with the mRNAs encoding master transcription regulators of curli and flagella synthesis, respectively, resulting in down-regulation and up-regulation of these important cell surface structures. In this study, we demonstrate that McaS activates synthesis of the exopolysaccharide β-1,6 N-acetyl-D-glucosamine (PGA) by binding the global RNA-binding protein CsrA, a negative regulator of pgaA translation. The McaS RNA bears at least two CsrA-binding sequences, and inactivation of these sites compromises CsrA binding, PGA regulation, and biofilm formation. Moreover, ectopic McaS expression leads to induction of two additional CsrA-repressed genes encoding diguanylate cyclases. Collectively, our study shows that McaS is a dual-function sRNA with roles in the two major post-transcriptional regulons controlled by the RNA-binding proteins Hfq and CsrA.