CsrA regulation via binding to the base-pairing small RNA Spot 42

The carbon storage regulator system and base-pairing small RNAs (sRNAs) represent two predominant modes of bacterial post-transcriptional regulation, which globally influence gene expression. Binding of CsrA protein to the 5' UTR or initial mRNA coding sequences can affect translation, RNA stability, and/or transcript elongation. Base-pairing sRNAs also regulate these processes, often requiring assistance from the RNA chaperone Hfq. Transcriptomics studies in Escherichia coli have identified many new CsrA targets, including Spot 42 and other base-pairing sRNAs. Spot 42 synthesis is repressed by cAMP-CRP, induced by the presence of glucose, and Spot 42 post-transcriptionally represses operons that facilitate metabolism of nonpreferred carbon sources. CsrA activity is also increased by glucose via effects on CsrA sRNA antagonists, CsrB/C. Here, we elucidate a mechanism wherein CsrA binds to and protects Spot 42 sRNA from RNase E-mediated cleavage. This protection leads to enhanced repression of srlA by Spot 42, a gene required for sorbitol uptake. A second, independent mechanism by which CsrA represses srlA is by binding to and inhibiting translation of srlM mRNA, encoding a transcriptional activator of srlA. Our findings demonstrate a novel means of regulation, by CsrA binding to a sRNA, and indicate that such interactions can help to shape complex bacterial regulatory circuitry.