CsrA

CsrA, short for carbon storage regulator A, is a small, global RNA-binding protein found in many bacteria. It acts as a master regulator of post-transcriptional gene expression, influencing metabolism, motility, biofilm formation, and virulence. The protein is typically a homodimer of ~7 kDa subunits that recognizes RNA sequences containing GGA motifs, usually within the 5' untranslated region or near the start codon of target mRNAs. Binding of CsrA to these sites often blocks ribosome access to the mRNA, repressing translation, and can also alter mRNA stability. As a result, CsrA can both repress and, in some contexts, stabilize transcripts.

CsrA activity is tightly controlled by regulatory small RNAs, notably CsrB and CsrC in Escherichia coli and related species. These sRNAs contain multiple CsrA-binding motifs and sequester CsrA, reducing its availability to target mRNAs. The relative levels of CsrA and its sRNA antagonists respond to growth conditions and environmental cues. In many bacteria, the turnover of CsrB/CsrC is promoted by the global regulator CsrD, linking external signals to CsrA activity.

Biological roles attributed to CsrA include coordination of carbon and glycogen metabolism, regulation of glucose uptake, flagellar synthesis, and biofilm development; in pathogenic bacteria, CsrA influences virulence gene expression and host interaction. The CsrA system is conserved across diverse Gram-negative bacteria, with functional homologs often called RsmA or RsmE in other species. Understanding CsrA provides insight into bacterial adaptation strategies through post-transcriptional regulation and small RNA–mediated signaling.