How bacteria translate proteins from structurally blocked mRNAs

Bacterial ribosomes need a single-stranded ribosome binding site (RBS) to initiate protein synthesis, while a stable RNA structure blocks initiation. Paradoxically, structured mRNAs can nevertheless be translated efficiently. Researchers at the University of Uppsala have now elucidated the anatomy of a site "pending" and its requirements, in order to overcome the problems of RNA structure for the translation.

The synthesis of bacterial proteins has been studied for decades. Ribosomes must have access to a single-stranded RBS system in order to initiate translation. However, some mRNAs with stably structured RBS regions are efficiently translated. About 25 years ago, Dutch researchers proposed a new mechanism to take into account this phenomenon, the "ribosome eve": a ribosome binds to an accessible unstructured region, waits for a moment, then goes to the RBS when its structure opens temporarily.

In this new study, published in Proceedings of the National Academy of SciencesUppsala University researchers unveiled the anatomy of a pending site and reported on the key role of S1 ribosomal protein in this process. S1 binds to a rescue site consisting of two elements, a single-stranded region and, unexpectedly, a short hairpin of RNA. The standby link allows the ribosome to move into the downstream RNA structure and access blocked RBS.

"We felt that it was time to understand exactly what an emergency site looked like and what to do to make it work." The standby is an old idea that was lacking evidence here. direct sound, "says Cédric Romilly, first author of the study.

Following studies conducted for several years by the Wagner group, they studied a short mRNA encoding a toxin, TisB. The translation of this protein depends entirely on a reserve site located more than 100 nucleotides upstream of the stable and inaccessible RBS structure. Using sophisticated biochemical methods, such as anisotropy of fluorescence and UV / RNA fingerprinting, researchers were able to catch the ribosome at the rescue site. The experiments show that it is the S1 protein that guides the ribosome to the reserve site, thus possibly favoring the opening of the downstream RNA structure to access the TBS RBS.

"It has really been a tour de force, but it is good to finally understand the anatomy of a real reserve site," says Professor E. Gerhart H. Wagner, lead author of the. study.