Molecular Mimicry in Translational Regulation: The Case of Ribosomal Protein S15

Abstract

Ribosomal protein S15 is highly conserved among prokaryotes. It plays a pivotal role in the assembly of the central domain of the small ribosomal subunit and regulates its own expression by a feedback mechanism at the translational level. The protein recognizes two RNA targets (rRNA and mRNA) that share only partial similarity. Its interaction with 16S rRNA has been fully characterized, while mRNA interactions and regulatory mechanisms have been extensively studied in E. coli and in T. thermophilus. Recently, we have characterized which aminoacids are involved in E. coli mRNA recognition, using an in vivo assay allowing to identify S15 mutations affecting the S15-mRNA interactions without altering 30S subunit assembly. Here, we address the following questions: Are common determinants used by S15 to recognize its rRNA and mRNA targets? What is the extent of molecular mimicry? Is the regulatory mechanism conserved? Our results indicate that specific recognition of mRNA and rRNA relies on both mimicry and site differentiation. They also highlight the high plasticity of RNA to adapt to evolutionary constraints.

The role of mRNA structure in translational control in bacteria

“…It was proposed that the entrapment mechanism would sustain rather weak binding affinities of protein-mRNA complex. 18,19 In agreement with this observation, the binding affinity of EcS15 is about one order of magnitude lower than that for its 16S rRNA binding site and of TtS15 for its mRNA target. In B. stearothermophilus (Fig.…”

Multiple ways to regulate translation initiation in bacteria: Mechanisms, regulatory circuits, dynamics

“…E. coli S15 (EcS15; Fig. 2B) binds to the pseudoknot recognizing a GeU/GeC motif similar to the 16S rRNA and other elements of the pseudoknot that are not present in the rRNA [89]. The complex is less stable (K d~2 00 nM) than in T. thermophilus case.…”

“…The complex is less stable (K d~2 00 nM) than in T. thermophilus case. Part of the rpsO RBS, including the SD sequence, is located in a large loop accessible for 30S interaction [88,89]. As a result, the SDeaSD helix anchors the EcS15erpsO mRNA complex on the platform of the 30S but translation is inhibited (Fig.…”

A structural view of translation initiation in bacteria

“…The cryo-EM structure of the 30S pre-IC was recently determined [21] taking advantage of the fact that structured mRNAs can be stabilized on the 30S if the accommodation step is blocked. This mechanism, called entrapment, is used by E. coli ribosomal proteins S15 and S4 to repress the translation of their own mRNAs and consequently of their whole operons [41,42]. The structure of the 30S pre-IC (i.e., in the presence of the repressor protein S15) shows that the pre-accommodated mRNA is bound in its folded state on the platform of the ribosome ( Fig.…”