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Global analysis of translation termination in E. coli using release factor manipulations

ABSTRACT: Terminating protein translation accurately and efficiently is critical for both protein fidelity and ribosome recycling for continued translation. The three bacterial release factors (RFs) play key roles: RF1 and 2 recognize stop codons and terminate translation; and RF3 promotes disassociation of bound release factors. Probing release factors mutations with reporter constructs containing programmed frameshifting sequences or premature stop codons had revealed a propensity for readthrough or frameshifting at these specific sites, but their effects on translation genome-wide have not been examined. We performed ribosome profiling on a set of isogenic strains with well-characterized release factor mutations to determine how they alter translation globally. Consistent with their known defects, strains with increasingly severe release factor defects exhibit increasingly severe accumulation of ribosomes over stop codons, indicative of an increased duration of the termination/release phase of translation. Release factor mutant strains also exhibit increased occupancy in the region following the stop codon at a significant number of genes. Our global analysis revealed that, as expected, translation termination is generally efficient and accurate, but that at a significant number of genes (≥ 50) the ribosome signature after the stop codon is suggestive of translation past the stop codon. Even native E. coli K-12 exhibits the ribosome signature suggestive of protein extension, especially at UGA codons, which rely exclusively on the reduced function RF2 allele of the K-12 strain for termination. Deletion of RF3 increases the severity of the defect. We unambiguously demonstrate readthrough and frameshifting protein extensions and their further accumulation in mutant strains for a few select cases. In addition to enhancing recoding, ribosome accumulation over stop codons disrupts attenuation control of biosynthetic operons, and may alter expression of some overlapping genes. Together, these functional alterations may either augment the protein repertoire or produce deleterious proteins.

ORGANISM(S): Escherichia coli

PROVIDER: GSE88725 | GEO | 2017/03/09

SECONDARY ACCESSION(S): PRJNA348358

REPOSITORIES: GEO

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Project description:Proline-rich antimicrobial peptide apidaecin (Api) inhibits bacterial protein synthesis in a distinctive way. In vitro biochemical and structural studies showed that Api binds in the nascent peptide exit tunnel of the ribosome that has completed translation of the gene and has released the newly-synthesized protein. Once in the tunnel, Api traps the release factors on the post-termination ribosome leading to cessation of translation. The mode of Api action in the bacterial cell, however, had remained unknow. By analyzing distribution of ribosomes on mRNAs in Api-treated cells using Ribo-seq approach we uncovered a range of effects that stem from the unique mechanism of Api action. Exposure of bacterial cells to Api results in arrest of translation at stop codons, likely in a post-release state, due to the immediate Api action, as well as in a pre-release state due to the depletion of available release factors. In addition, arrest of translation at the end of the open reading frame (ORF) leads to a pronounced queuing of the translating ribosomes behind the one arrested at the stop codon. One of the major consequences of the Api action is a dramatically increased stop codon bypass by the ribosomes paused in a pre-release state, leading to accumulation of proteins with C-terminal extensions, as confirmed by whole-cell proteomics analysis. Stop codon bypass, that occurs either in 0-frame, by misincorporation of a near-cognate aminoacyl-tRNA at the stop codon, or via frameshifting allows for a fraction of the translating ribosomes to reach the 3’ ends of mRNA transcripts. The pervasive stalling of pre-release ribosomes at the stop codons and at the RNA transcripts ends triggers activation of the cellular ribosome rescue systems which, likely due to Api action as well, remain ineffective. While major Api effects are directed towards the ribosome at the end of the ORFs, cells exposed to Api show a somewhat increased ribosome occupancy of the start codons. Understanding the unique mode of Api to inhibit translation termination in the cell can help envisioning the development of treatments for genetic diseases and research tools for genome exploration.

Project description:Translation of mRNA into a polypeptide is terminated when the release factor eRF1 recognizes a UAA, UAG, or UGA stop codon in the ribosomal A site and stimulates nascent peptide release. However, stop codon readthrough can occur when a near-cognate tRNA outcompetes eRF1 in decoding the stop codon, resulting in the continuation of the elongation phase of protein synthesis. At the end of a conventional mRNA coding region readthrough allows translation into the mRNA 3’-UTR. Previous studies with reporter systems have shown that the efficiency of termination or readthrough is modulated by cis-acting elements other than stop codon identity, including two nucleotides 5’ of the stop codon, six nucleotides 3’ of the stop codon in the ribosomal mRNA channel, and stem-loop structures in the mRNA 3’-UTR. It is unknown whether these elements are important at a genome-wide level and whether other mRNA features proximal to the stop codon significantly affect termination and readthrough efficiencies in vivo. Accordingly, we carried out ribosome profiling analyses of yeast cells expressing wild-type or temperature-sensitive eRF1 and developed bioinformatics strategies to calculate readthrough efficiency, and to identify mRNA and peptide features which influence that efficiency. We found that the stop codon (nt +1 to +3), the nucleotide after it (nt +4), the codon in the P site (nt -3 to -1), and 3’-UTR length are the most influential features in the control of readthrough efficiency, while nts +5 to +9 and mRNA secondary structure in the 3’-UTR had milder effects. Additionally, we found low readthrough genes to have shorter 3’-UTRs compared to high readthrough genes in cells with thermally inactivated eRF1, while this trend was reversed in wild-type cells. Together, our results demonstrated the general roles of known regulatory elements in genome-wide regulation and identified several new mRNA or peptide features important for translation termination and readthrough.

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Global analysis of translation termination in E. coli using release factor manipulations

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