The anti-Shine-Dalgarno sequence drives translational pausing and codon choice in bacteria.

Department of Cellular and Molecular Pharmacology, Howard Hughes Medical Institute, University of California, San Francisco, California 94158, USA.
Nature (Impact Factor: 38.6). 03/2012; 484(7395):538-41. DOI: 10.1038/nature10965
Source: PubMed

ABSTRACT Protein synthesis by ribosomes takes place on a linear substrate but at non-uniform speeds. Transient pausing of ribosomes can affect a variety of co-translational processes, including protein targeting and folding. These pauses are influenced by the sequence of the messenger RNA. Thus, redundancy in the genetic code allows the same protein to be translated at different rates. However, our knowledge of both the position and the mechanism of translational pausing in vivo is highly limited. Here we present a genome-wide analysis of translational pausing in bacteria by ribosome profiling--deep sequencing of ribosome-protected mRNA fragments. This approach enables the high-resolution measurement of ribosome density profiles along most transcripts at unperturbed, endogenous expression levels. Unexpectedly, we found that codons decoded by rare transfer RNAs do not lead to slow translation under nutrient-rich conditions. Instead, Shine-Dalgarno-(SD)-like features within coding sequences cause pervasive translational pausing. Using an orthogonal ribosome possessing an altered anti-SD sequence, we show that pausing is due to hybridization between the mRNA and 16S ribosomal RNA of the translating ribosome. In protein-coding sequences, internal SD sequences are disfavoured, which leads to biased usage, avoiding codons and codon pairs that resemble canonical SD sites. Our results indicate that internal SD-like sequences are a major determinant of translation rates and a global driving force for the coding of bacterial genomes.

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    ABSTRACT: MOTIVATION: Ribosome profiling is a new technique that allows monitoring locations of translating ribosomes on mRNA at a whole transcriptome level. A recent ribosome profiling study demonstrated that internal Shine-Dalgarno (SD) sequences have a major global effect on translation rates in bacteria: ribosomes pause at SD sites in mRNA. Therefore it is important to understand how SD sites effect mRNA movement through the ribosome and generation of ribosome footprints. RESULTS: Here, we provide evidence that in addition to pausing effect, internal SD sequences induce a caterpillar like movement of mRNA through the ribosome cavity. Once an SD site binds to the ribosome, it remains attached to it while the ribosome decodes a few subsequent codons. This leads to asymmetric progressive elongation of ribosome footprints at the 3' end. It is likely that internal SD sequences induce a pause not on a single, but on several adjacent codons. This finding is important for our understanding of mRNA movement through the ribosome and also should facilitate interpretation of ribosome profiling data. CONTACT:
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