CsrARsmA

CsrA/RsmA refers to a family of small, RNA-binding proteins that act as global regulators of gene expression at the post-transcriptional level in bacteria. The best known representatives are CsrA in Escherichia coli and Klebsiella pneumoniae and RsmA in Pseudomonas species; these proteins are highly conserved and function as homodimers.

Mechanism: CsrA/RsmA binds to specific sequence motifs, often GGA, located in the 5' untranslated regions of target mRNAs near ribosome binding sites. Binding typically blocks ribosome access and represses translation, and in some cases promotes mRNA degradation or modulates mRNA stability. The full regulatory effect arises from the combined action on numerous targets, including genes involved in metabolism, motility, storage, and virulence.

Regulation by small RNAs: The activity of CsrA/RsmA is antagonized by small regulatory RNAs, including CsrB and CsrC in Enterobacteriaceae and RsmY/RsmZ (and related) in Pseudomonas. These RNAs contain multiple CsrA-binding motifs and sequester the protein, reducing its availability to repress targets. The transcription of these sRNAs is controlled by two-component systems such as BarA/UvrY or GacS/GacA, linking CsrA/RsmA activity to environmental cues.

Structure and distribution: CsrA/RsmA proteins are typically ~7 kDa per monomer and form homodimers. The family is widespread across bacteria, with multiple species harboring additional homologs (e.g., CsrA1, CsrA2, RsmA/E variants).

Significance: As a post-transcriptional regulator, CsrA/RsmA coordinates diverse cellular processes, influencing metabolism, motility, biofilm formation, and virulence. Owing to their central role, these proteins are a common focus in studies of bacterial gene regulation.