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. 2020 Jul 10;7:127. doi: 10.3389/fmolb.2020.00127

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Copyright © 2020 Sobrero and Valverde.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Direct and indirect regulatory effects of CsrA binding to a mRNA. (1) Direct translational repression. In most cases, CsrA binds to target sites around or within the Shine-Dalgarno region, and as a consequence it outcompetes binding of the small ribosomal subunit and its engagement in translation initiation. (2) Indirect translational repression. CsrA binds to target sites in the 5′-UTR and induces a conformational change that exposes the Shine-Dalgarno region to base-pairing by a small regulatory RNA, thus impeding access of the ribosome. (3) Indirect control of mRNA stability. CsrA binds to a target site that upon refolding, impedes access of a ribonuclease. (4) Indirect control of transcriptional termination. CsrA binds to a target site in the mRNA and induces a conformational change that exposes Rho-utilization sequences that can now be exploited by Rho protein to stop transcription elongation of the mRNA. (5) In the absence of CsrA (either due to a mutation that abolishes its function or due to sequestration by CsrA-antagonists, as depicted in Figure 4), a ribosome can recognize the Shine-Dalgarno sequence and initiate mRNA translation. Usually, translationally active mRNAs are protected from the action of ribonucleases.