Abstract
Post-transcriptional regulation mediated by regulatory small RNAs (sRNAs) has risen as a key player in fine-tuning gene expression in response to environmental stimuli. Here, we show that, in Salmonella enterica, the central metabolic regulator CRP-cAMP differentially regulates the sRNAs CsrB and CsrC in a growth phase-dependent manner. While CsrB expression remains unchanged during growth, CsrC displays a growth phase-dependent expression profile, being weakly expressed at the logarithmic growth phase and induced upon entry into stationary phase. We show that CRP-cAMP contributes to the expression pattern of CsrC by repressing its expression during the logarithmic growth phase. The CRP-cAMP mediated repression of CsrC is independent of SirA, a known transcriptional CsrB/CsrC activator. We further show that the sRNA Spot 42, which is derepressed in a Δcrp strain, upregulates CsrC during logarithmic growth. We propose a model where the growth-dependent regulation of CsrC is sustained by the CRP-cAMP-mediated repression of Spot 42. Together, our data point toward a differential regulation of the sRNAs CsrB and CsrC in response to environmental stimuli, leading to fine-tuning of gene expression via the sequestration of the RNA-binding protein CsrA.
Highlights
Bacteria need to adapt rapidly to changing environmental conditions, which is crucial for pathogenic bacteria during the process of infection
No effect on CsrB regulation by CRPcAMP was observed. These results were unexpected since it was previously described that CRP-cAMP acts as an activator of both CsrB and CsrC expression when Salmonella is grown on Lysogeny broth (LB) agar media (Teplitski et al, 2006)
CRP-cAMP seem to be required for activation of the expression of CsrB and CsrC in solid media, while it acts as a repressor of CsrC at logarithmic growth phase
Summary
Bacteria need to adapt rapidly to changing environmental conditions, which is crucial for pathogenic bacteria during the process of infection. Among the described post-transcriptional regulation mechanisms, a prominent role has been attributed to sRNAs that modulate gene expression primarily via binding to target mRNAs (Hör et al, 2020). SRNAs can regulate gene expression at the post-translational level, such as the two sRNAs CsrB and CsrC. These sRNAs contain several stem loop structures with GGA motifs in their loop regions, enabling them to bind CsrA. CsrA is a widely conserved RNA-binding protein that inhibits translation by binding to GGA motifs around the ribosome-binding site of target mRNAs (Romeo and Babitzke, 2018).
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