CsrA Participates in a PNPase Autoregulatory Mechanism by Selectively Repressing Translation of pnp Transcripts That Have Been Previously Processed by RNase III and PNPase.

Abstract

Csr is a conserved global regulatory system that represses or activates gene expression posttranscriptionally. CsrA of Escherichia coli is a homodimeric RNA binding protein that regulates transcription elongation, translation initiation, and mRNA stability by binding to the 5' untranslated leader or initial coding sequence of target transcripts. pnp mRNA, encoding the 3' to 5' exoribonuclease polynucleotide phosphorylase (PNPase), was previously identified as a CsrA target by transcriptome sequencing (RNA-seq). Previous studies also showed that RNase III and PNPase participate in a pnp autoregulatory mechanism in which RNase III cleavage of the untranslated leader, followed by PNPase degradation of the resulting 5' fragment, leads to pnp repression by an undefined translational repression mechanism. Here we demonstrate that CsrA binds to two sites in pnp leader RNA but only after the transcript is fully processed by RNase III and PNPase. In the absence of processing, both of the binding sites are sequestered in an RNA secondary structure, which prevents CsrA binding. The CsrA dimer bridges the upstream high-affinity site to the downstream site that overlaps the pnp Shine-Dalgarno sequence such that bound CsrA causes strong repression of pnp translation. CsrA-mediated translational repression also leads to a small increase in the pnp mRNA decay rate. Although CsrA has been shown to regulate translation and mRNA stability of numerous genes in a variety of organisms, this is the first example in which prior mRNA processing is required for CsrA-mediated regulation. CsrA protein represses translation of numerous mRNA targets, typically by binding to multiple sites in the untranslated leader region preceding the coding sequence. We found that CsrA represses translation of pnp by binding to two sites in the pnp leader transcript but only after it is processed by RNase III and PNPase. Processing by these two ribonucleases alters the mRNA secondary structure such that it becomes accessible to the ribosome for translation as well as to CsrA. As one of the CsrA binding sites overlaps the pnp ribosome binding site, bound CsrA prevents ribosome binding. This is the first example in which regulation by CsrA requires prior mRNA processing and should link pnp expression to conditions affecting CsrA activity.