1
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Lahry K, Datta M, Varshney U. Genetic analysis of translation initiation in bacteria: An initiator tRNA-centric view. Mol Microbiol 2024. [PMID: 38410838 DOI: 10.1111/mmi.15243] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2023] [Revised: 02/03/2024] [Accepted: 02/09/2024] [Indexed: 02/28/2024]
Abstract
Translation of messenger RNA (mRNA) in bacteria occurs in the steps of initiation, elongation, termination, and ribosome recycling. The initiation step comprises multiple stages and uses a special transfer RNA (tRNA) called initiator tRNA (i-tRNA), which is first aminoacylated and then formylated using methionine and N10 -formyl-tetrahydrofolate (N10 -fTHF), respectively. Both methionine and N10 -fTHF are produced via one-carbon metabolism, linking translation initiation with active cellular metabolism. The fidelity of i-tRNA binding to the ribosomal peptidyl-site (P-site) is attributed to the structural features in its acceptor stem, and the highly conserved three consecutive G-C base pairs (3GC pairs) in the anticodon stem. The acceptor stem region is important in formylation of the amino acid attached to i-tRNA and in its initial binding to the P-site. And, the 3GC pairs are crucial in transiting the i-tRNA through various stages of initiation. We utilized the feature of 3GC pairs to investigate the nuanced layers of scrutiny that ensure fidelity of translation initiation through i-tRNA abundance and its interactions with the components of the translation apparatus. We discuss the importance of i-tRNA in the final stages of ribosome maturation, as also the roles of the Shine-Dalgarno sequence, ribosome heterogeneity, initiation factors, ribosome recycling factor, and coevolution of the translation apparatus in orchestrating a delicate balance between the fidelity of initiation and/or its leakiness to generate proteome plasticity in cells to confer growth fitness advantages in response to the dynamic nutritional states.
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Affiliation(s)
- Kuldeep Lahry
- Department of Microbiology and Cell Biology, Indian Institute of Science, Bengaluru, India
| | - Madhurima Datta
- Department of Microbiology and Cell Biology, Indian Institute of Science, Bengaluru, India
| | - Umesh Varshney
- Department of Microbiology and Cell Biology, Indian Institute of Science, Bengaluru, India
- Jawaharlal Nehru Centre for Advanced Scientific Research, Bengaluru, India
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2
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Chloroplasts evolved an additional layer of translational regulation based on non-AUG start codons for proteins with different turnover rates. Sci Rep 2023; 13:896. [PMID: 36650197 PMCID: PMC9845219 DOI: 10.1038/s41598-022-27347-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/13/2022] [Accepted: 12/30/2022] [Indexed: 01/18/2023] Open
Abstract
Chloroplasts have evolved from photosynthetic cyanobacteria-like progenitors through endosymbiosis. The chloroplasts of present-day land plants have their own transcription and translation systems that show several similarities with prokaryotic organisms. A remarkable feature of the chloroplast translation system is the use of non-AUG start codons in the protein synthesis of certain genes that are evolutionarily conserved from Algae to angiosperms. However, the biological significance of such use of non-AUG codons is not fully understood. The present study was undertaken to unravel the significance of non-AUG start codons in vivo using the chloroplast genetic engineering approach. For this purpose, stable transplastomic tobacco plants expressing a reporter gene i.e. uidA (GUS) under four different start codons (AUG/UUG/GUG/CUG) were generated and β-glucuronidase (GUS) expression was compared. To investigate further the role of promoter sequences proximal to the start codon, uidA was expressed under two different chloroplast gene promoters psbA and psbC that use AUG and a non-AUG (GUG) start codons, respectively, and also showed significant differences in the DNA sequence surrounding the start codon. Further, to delineate the role of RNA editing that creates AUG start codon by editing non-AUG codons, if any, which is another important feature of the chloroplast transcription and translation system, transcripts were sequenced. In addition, a proteomic approach was used to identify the translation initiation site(s) of GUS and the N-terminal amino acid encoded when expressed under different non-AUG start codons. The results showed that chloroplasts use non-AUG start codons in combination with the translation initiation site as an additional layer of gene regulation to over-express proteins that are required at high levels due to their high rates of turnover.
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3
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Datta M, Singh J, Modak MJ, Pillai M, Varshney U. Systematic evolution of initiation factor 3 and the ribosomal protein uS12 optimizes Escherichia coli growth with an unconventional initiator tRNA. Mol Microbiol 2021; 117:462-479. [PMID: 34889476 DOI: 10.1111/mmi.14861] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2021] [Revised: 12/07/2021] [Accepted: 12/07/2021] [Indexed: 11/28/2022]
Abstract
The anticodon stem of initiator tRNA (i-tRNA) possesses the characteristic three consecutive GC base pairs (G29:C41, G30:C40, and G31:C39 abbreviated as GC/GC/GC or 3GC pairs) crucial to commencing translation. To understand the importance of this highly conserved element, we isolated two fast-growing suppressors of Escherichia coli sustained solely on an unconventional i-tRNA (i-tRNAcg/GC/cg ) having cg/GC/cg sequence instead of the conventional GC/GC/GC. Both suppressors have the common mutation of V93A in initiation factor 3 (IF3), and additional mutations of either V32L (Sup-1) or H76L (Sup-2) in small subunit ribosomal protein 12 (uS12). The V93A mutation in IF3 was necessary for relaxed fidelity of i-tRNA selection to sustain on i-tRNAcg/GC/cg though with a retarded growth. Subsequent mutations in uS12 salvaged the retarded growth by enhancing the fidelity of translation. The H76L mutation in uS12 showed better fidelity of i-tRNA selection. However, the V32L mutation compensated for the deficient fidelity of i-tRNA selection by ensuring an efficient fidelity check by ribosome recycling factor (RRF). We reveal unique genetic networks between uS12, IF3 and i-tRNA in initiation and between uS12, elongation factor-G (EF-G), RRF, and Pth (peptidyl-tRNA hydrolase) which, taken together, govern the fidelity of translation in bacteria.
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Affiliation(s)
- Madhurima Datta
- Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, India
| | - Jitendra Singh
- Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, India
| | - Mamata Jayant Modak
- Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, India
| | - Maalavika Pillai
- Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, India
| | - Umesh Varshney
- Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, India.,Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore, India
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4
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Mehner-Breitfeld D, Rathmann C, Riedel T, Just I, Gerhard R, Overmann J, Brüser T. Evidence for an Adaptation of a Phage-Derived Holin/Endolysin System to Toxin Transport in Clostridioides difficile. Front Microbiol 2018; 9:2446. [PMID: 30405545 PMCID: PMC6200909 DOI: 10.3389/fmicb.2018.02446] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/27/2018] [Accepted: 09/25/2018] [Indexed: 12/16/2022] Open
Abstract
The pathogenicity locus (PaLoc) of Clostridioides difficile usually comprises five genes (tcdR, tcdB, tcdE, tcdA, tcdC). While the proteins TcdA and TcdB represent the main toxins of this pathogen, TcdR and TcdC are involved in the regulation of their production. TcdE is a holin family protein, members of which are usually involved in the transport of cell wall-degrading enzymes (endolysins) for phage-induced lysis. In the past, TcdE has been shown to contribute to the release of TcdA and TcdB, but it is unclear whether it mediates a specific transport or rather a lysis of cells. TcdE of C. difficile strains analyzed so far can be produced in three isoforms that are initiated from distinct N-terminal ATG codons. When produced in Escherichia coli, we found that the longest TcdE isoform had a moderate effect on cell growth, whereas the shortest isoform strongly induced lysis. The effect of the longest isoform was inhibitory for cell lysis, implying a regulatory function of the N-terminal 24 residues. We analyzed the PaLoc sequence of 44 C. difficile isolates and found that four of these apparently encode only the short TcdE isoforms, and the most closely related holins from C. difficile phages only possess one of these initiation codons, indicating that an N-terminal extension of TcdE evolved in C. difficile. All PaLoc sequences comprised also a conserved gene encoding a short fragment of an endolysin remnant of a phage holin/endolysin pair. We could produce this peptide, which we named TcdL, and demonstrated by bacterial two-hybrid analysis a self-interaction and an interaction with TcdB that might serve to mediate TcdE-dependent transport.
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Affiliation(s)
| | - Claudia Rathmann
- Institute of Microbiology, Leibniz Universität Hannover, Hanover, Germany
| | - Thomas Riedel
- Department of Microbial Ecology and Diversity Research, Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures, Braunschweig, Germany.,German Center for Infection Research, Partner Site Hannover-Braunschweig, Braunschweig, Germany
| | - Ingo Just
- Institute of Toxicology, Hannover Medical School, Hanover, Germany
| | - Ralf Gerhard
- Institute of Toxicology, Hannover Medical School, Hanover, Germany
| | - Jörg Overmann
- Department of Microbial Ecology and Diversity Research, Leibniz Institute DSMZ-German Collection of Microorganisms and Cell Cultures, Braunschweig, Germany.,German Center for Infection Research, Partner Site Hannover-Braunschweig, Braunschweig, Germany
| | - Thomas Brüser
- Institute of Microbiology, Leibniz Universität Hannover, Hanover, Germany
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5
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Ayyub SA, Dobriyal D, Shah RA, Lahry K, Bhattacharyya M, Bhattacharyya S, Chakrabarti S, Varshney U. Coevolution of the translational machinery optimizes initiation with unusual initiator tRNAs and initiation codons in mycoplasmas. RNA Biol 2017; 15:70-80. [PMID: 28901843 DOI: 10.1080/15476286.2017.1377879] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022] Open
Abstract
Initiator tRNAs (i-tRNAs) are characterized by the presence of three consecutive GC base pairs (GC/GC/GC) in their anticodon stems in all domains of life. However, many mycoplasmas possess unconventional i-tRNAs wherein the highly conserved sequence of GC/GC/GC is represented by AU/GC/GC, GC/GC/GU or AU/GC/GU. These mycoplasmas also tend to preferentially utilize non-AUG initiation codons. To investigate if initiation with the unconventional i-tRNAs and non-AUG codons in mycoplasmas correlated with the changes in the other components of the translation machinery, we carried out multiple sequence alignments of genes encoding initiation factors (IF), 16S rRNAs, and the ribosomal proteins such as uS9, uS12 and uS13. In addition, the occurrence of Shine-Dalgarno sequences in mRNAs was analyzed. We observed that in the mycoplasmas harboring AU/GC/GU i-tRNAs, a highly conserved position of R131 in IF3, is represented by P, F or Y and, the conserved C-terminal tail (SKR) of uS9 is represented by the TKR sequence. Using the Escherichia coli model, we show that the change of R131 in IF3 optimizes initiation with the AU/GC/GU i-tRNAs. Also, the SKR to TKR change in uS9 was compatible with the R131P variation in IF3 for initiation with the AU/GC/GU i-tRNA variant. Interestingly, the mycoplasmas harboring AU/GC/GU i-tRNAs are also human pathogens. We propose that these mycoplasmas might have evolved a relaxed translational apparatus to adapt to the environment they encounter in the host.
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Affiliation(s)
- Shreya Ahana Ayyub
- a Department of Microbiology and Cell Biology , Indian Institute of Science , Bangalore , India
| | - Divya Dobriyal
- a Department of Microbiology and Cell Biology , Indian Institute of Science , Bangalore , India
| | - Riyaz Ahmad Shah
- a Department of Microbiology and Cell Biology , Indian Institute of Science , Bangalore , India
| | - Kuldeep Lahry
- a Department of Microbiology and Cell Biology , Indian Institute of Science , Bangalore , India
| | - Madhumita Bhattacharyya
- b Structural Biology and Bioinformatics Division , CSIR-Indian Institute of Chemical Biology , Kolkata , India
| | - Souvik Bhattacharyya
- a Department of Microbiology and Cell Biology , Indian Institute of Science , Bangalore , India
| | - Saikat Chakrabarti
- b Structural Biology and Bioinformatics Division , CSIR-Indian Institute of Chemical Biology , Kolkata , India
| | - Umesh Varshney
- a Department of Microbiology and Cell Biology , Indian Institute of Science , Bangalore , India.,c Jawaharlal Nehru Centre for Advanced Scientific Research , Jakkur , Bangalore , India
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6
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D'Lima NG, Khitun A, Rosenbloom AD, Yuan P, Gassaway BM, Barber KW, Rinehart J, Slavoff SA. Comparative Proteomics Enables Identification of Nonannotated Cold Shock Proteins in E. coli. J Proteome Res 2017; 16:3722-3731. [PMID: 28861998 PMCID: PMC5647875 DOI: 10.1021/acs.jproteome.7b00419] [Citation(s) in RCA: 19] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
![]()
Recent advances in mass spectrometry-based
proteomics have revealed
translation of previously nonannotated microproteins from thousands
of small open reading frames (smORFs) in prokaryotic and eukaryotic
genomes. Facile methods to determine cellular functions of these newly
discovered microproteins are now needed. Here, we couple semiquantitative
comparative proteomics with whole-genome database searching to identify
two nonannotated, homologous cold shock-regulated microproteins in Escherichia coli K12 substr. MG1655, as well as two
additional constitutively expressed microproteins. We apply molecular
genetic approaches to confirm expression of these cold shock proteins
(YmcF and YnfQ) at reduced temperatures and identify the noncanonical
ATT start codons that initiate their translation. These proteins are
conserved in related Gram-negative bacteria and are predicted to be
structured, which, in combination with their cold shock upregulation,
suggests that they are likely to have biological roles in the cell.
These results reveal that previously unknown factors are involved
in the response of E. coli to lowered
temperatures and suggest that further nonannotated, stress-regulated E. coli microproteins may remain to be found. More
broadly, comparative proteomics may enable discovery of regulated,
and therefore potentially functional, products of smORF translation
across many different organisms and conditions.
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Affiliation(s)
- Nadia G D'Lima
- Department of Chemistry, Yale University , New Haven, Connecticut 06520, United States.,Chemical Biology Institute, Yale University , West Haven, Connecticut 06516, United States
| | - Alexandra Khitun
- Department of Chemistry, Yale University , New Haven, Connecticut 06520, United States.,Chemical Biology Institute, Yale University , West Haven, Connecticut 06516, United States
| | - Aaron D Rosenbloom
- Department of Chemistry, Yale University , New Haven, Connecticut 06520, United States
| | - Peijia Yuan
- Department of Chemistry, Yale University , New Haven, Connecticut 06520, United States.,Chemical Biology Institute, Yale University , West Haven, Connecticut 06516, United States
| | - Brandon M Gassaway
- Department of Cellular and Molecular Physiology, Yale University , New Haven, Connecticut 06520, United States.,Systems Biology Institute, Yale University , West Haven, Connecticut 06511, United States
| | - Karl W Barber
- Department of Cellular and Molecular Physiology, Yale University , New Haven, Connecticut 06520, United States.,Systems Biology Institute, Yale University , West Haven, Connecticut 06511, United States
| | - Jesse Rinehart
- Department of Cellular and Molecular Physiology, Yale University , New Haven, Connecticut 06520, United States.,Systems Biology Institute, Yale University , West Haven, Connecticut 06511, United States
| | - Sarah A Slavoff
- Department of Chemistry, Yale University , New Haven, Connecticut 06520, United States.,Chemical Biology Institute, Yale University , West Haven, Connecticut 06516, United States.,Department of Molecular Biophysics and Biochemistry, Yale University , New Haven, Connecticut 06529, United States
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7
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Abstract
DNA exonucleases, enzymes that hydrolyze phosphodiester bonds in DNA from a free end, play important cellular roles in DNA repair, genetic recombination and mutation avoidance in all organisms. This article reviews the structure, biochemistry, and biological functions of the 17 exonucleases currently identified in the bacterium Escherichia coli. These include the exonucleases associated with DNA polymerases I (polA), II (polB), and III (dnaQ/mutD); Exonucleases I (xonA/sbcB), III (xthA), IV, VII (xseAB), IX (xni/xgdG), and X (exoX); the RecBCD, RecJ, and RecE exonucleases; SbcCD endo/exonucleases; the DNA exonuclease activities of RNase T (rnt) and Endonuclease IV (nfo); and TatD. These enzymes are diverse in terms of substrate specificity and biochemical properties and have specialized biological roles. Most of these enzymes fall into structural families with characteristic sequence motifs, and members of many of these families can be found in all domains of life.
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8
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Caban K, Gonzalez RL. The emerging role of rectified thermal fluctuations in initiator aa-tRNA- and start codon selection during translation initiation. Biochimie 2015; 114:30-8. [PMID: 25882682 DOI: 10.1016/j.biochi.2015.04.001] [Citation(s) in RCA: 10] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/27/2014] [Accepted: 04/02/2015] [Indexed: 11/30/2022]
Abstract
Decades of genetic, biochemical, biophysical, and structural studies suggest that the conformational dynamics of the translation machinery (TM), of which the ribosome is the central component, play a fundamental role in the mechanism and regulation of translation. More recently, single-molecule fluorescence resonance energy transfer (smFRET) studies have provided a unique and powerful approach for directly monitoring the real-time dynamics of the TM. Indeed, smFRET studies of the elongation stage of translation have significantly enriched our understanding of the mechanisms through which stochastic, thermally driven conformational fluctuations of the TM are exploited to drive and regulate the individual steps of translation elongation [1]. Beyond translation elongation, smFRET studies of the conformational dynamics of the initiation stage of translation offer great potential for providing mechanistic information that has thus far remained difficult or impossible to obtain using traditional methods. This is particularly true of the mechanisms through which the accuracy of initiator tRNA- and start codon selection is established during translation initiation. Given that translation initiation is a major checkpoint for regulating the translation of mRNAs, obtaining such mechanistic information holds great promise for our understanding of the translational regulation of gene expression. Here, we provide an overview of the bacterial translation initiation pathway, summarize what is known regarding the biochemical functions of the IFs, and discuss various new and exciting mechanistic insights that have emerged from several recently published smFRET studies of the mechanisms that guide initiator tRNA- and start codon selection during translation initiation. These studies provide a springboard for future investigations of the conformational dynamics of the more complex eukaryotic translation initiation pathway and mechanistic studies of the role of translational regulation of gene expression in human health and disease.
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Affiliation(s)
- Kelvin Caban
- Department of Chemistry, Columbia University, New York, NY 10027, USA
| | - Ruben L Gonzalez
- Department of Chemistry, Columbia University, New York, NY 10027, USA.
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9
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Firnberg E, Labonte JW, Gray JJ, Ostermeier M. A comprehensive, high-resolution map of a gene's fitness landscape. Mol Biol Evol 2014; 31:1581-92. [PMID: 24567513 PMCID: PMC4032126 DOI: 10.1093/molbev/msu081] [Citation(s) in RCA: 195] [Impact Index Per Article: 19.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/25/2022] Open
Abstract
Mutations are central to evolution, providing the genetic variation upon which selection acts. A mutation’s effect on the suitability of a gene to perform a particular function (gene fitness) can be positive, negative, or neutral. Knowledge of the distribution of fitness effects (DFE) of mutations is fundamental for understanding evolutionary dynamics, molecular-level genetic variation, complex genetic disease, the accumulation of deleterious mutations, and the molecular clock. We present comprehensive DFEs for point and codon mutants of the Escherichia coli TEM-1 β-lactamase gene and missense mutations in the TEM-1 protein. These DFEs provide insight into the inherent benefits of the genetic code’s architecture, support for the hypothesis that mRNA stability dictates codon usage at the beginning of genes, an extensive framework for understanding protein mutational tolerance, and evidence that mutational effects on protein thermodynamic stability shape the DFE. Contrary to prevailing expectations, we find that deleterious effects of mutation primarily arise from a decrease in specific protein activity and not cellular protein levels.
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Affiliation(s)
- Elad Firnberg
- Department of Chemical and Biomolecular Engineering, Johns Hopkins University
| | - Jason W Labonte
- Department of Chemical and Biomolecular Engineering, Johns Hopkins University
| | - Jeffrey J Gray
- Department of Chemical and Biomolecular Engineering, Johns Hopkins University
| | - Marc Ostermeier
- Department of Chemical and Biomolecular Engineering, Johns Hopkins University
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10
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Belov OV, Chuluunbaatar O, Kapralov MI, Sweilam NH. The role of the bacterial mismatch repair system in SOS-induced mutagenesis: a theoretical background. J Theor Biol 2013; 332:30-41. [PMID: 23643530 DOI: 10.1016/j.jtbi.2013.04.026] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/29/2012] [Revised: 04/20/2013] [Accepted: 04/22/2013] [Indexed: 10/26/2022]
Abstract
A theoretical study is performed of the possible role of the methyl-directed mismatch repair system in the ultraviolet-induced mutagenesis of Escherichia coli bacterial cells. For this purpose, mathematical models of the SOS network, translesion synthesis and mismatch repair are developed. Within the proposed models, the key pathways of these repair systems were simulated on the basis of modern experimental data related to their mechanisms. Our model approach shows a possible mechanistic explanation of the hypothesis that the bacterial mismatch repair system is responsible for attenuation of mutation frequency during ultraviolet-induced SOS response via removal of the nucleotides misincorporated by DNA polymerase V (the UmuD'2C complex).
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Affiliation(s)
- Oleg V Belov
- Laboratory of Radiation Biology, Joint Institute for Nuclear Research, 6 Joliot-Curie Street, 141980 Dubna, Moscow Region, Russia.
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11
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Qin D, Liu Q, Devaraj A, Fredrick K. Role of helix 44 of 16S rRNA in the fidelity of translation initiation. RNA (NEW YORK, N.Y.) 2012; 18:485-95. [PMID: 22279149 PMCID: PMC3285936 DOI: 10.1261/rna.031203.111] [Citation(s) in RCA: 27] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/01/2011] [Accepted: 12/08/2011] [Indexed: 05/30/2023]
Abstract
The molecular mechanisms that govern translation initiation to ensure accuracy remain unclear. Here, we provide evidence that the subunit-joining step of initiation is controlled in part by a conformational change in the 1408 region of helix h44. First, chemical probing of 30S initiation complexes formed with either a cognate (AUG) or near-cognate (AUC) start codon shows that an IF1-dependent enhancement at A1408 is reduced in the presence of AUG. This change in reactivity is due to a conformational change rather than loss of IF1, because other portions of the IF1 footprint are unchanged and high concentrations of IF1 fail to diminish the reactivity difference seen at A1408. Second, mutations in h44 such as A1413C stimulate 50S docking and cause reduced reactivity at A1408. Third, streptomycin, which has been shown by Rodnina and coworkers to stimulate 50S docking by reversing the inhibitory effects of IF1, also causes reduced reactivity at A1408. Collectively, these data support a model in which IF1 alters the A1408 region of h44 in a way that makes 50S docking unfavorable, and canonical codon-anticodon pairing in the P site restores h44 to a docking-favorable conformation. We also find that, in the absence of factors, the cognate 30S•AUG•fMet-tRNA ternary complex is >1000-fold more stable than the near-cognate 30S•AUC•fMet-tRNA complex. Hence, the selectivity of ternary complex formation is inherently high, exceeding that of initiation in vivo by more than 10-fold.
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MESH Headings
- Codon, Initiator/genetics
- Codon, Initiator/metabolism
- Escherichia coli/genetics
- Escherichia coli/metabolism
- Mutation
- Nucleic Acid Conformation
- Peptide Chain Initiation, Translational/drug effects
- RNA, Messenger/metabolism
- RNA, Ribosomal, 16S/chemistry
- RNA, Ribosomal, 16S/genetics
- RNA, Ribosomal, 16S/metabolism
- RNA, Transfer/genetics
- RNA, Transfer/metabolism
- Ribosome Subunits, Small, Bacterial/metabolism
- Streptomycin/pharmacology
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Affiliation(s)
- Daoming Qin
- Department of Microbiology, Ohio State Biochemistry Program, and Center for RNA Biology, The Ohio State University, Columbus, Ohio 43210, USA
| | - Qi Liu
- Department of Microbiology, Ohio State Biochemistry Program, and Center for RNA Biology, The Ohio State University, Columbus, Ohio 43210, USA
| | - Aishwarya Devaraj
- Department of Microbiology, Ohio State Biochemistry Program, and Center for RNA Biology, The Ohio State University, Columbus, Ohio 43210, USA
| | - Kurt Fredrick
- Department of Microbiology, Ohio State Biochemistry Program, and Center for RNA Biology, The Ohio State University, Columbus, Ohio 43210, USA
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12
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Betney R, de Silva E, Krishnan J, Stansfield I. Autoregulatory systems controlling translation factor expression: thermostat-like control of translational accuracy. RNA (NEW YORK, N.Y.) 2010; 16:655-63. [PMID: 20185543 PMCID: PMC2844614 DOI: 10.1261/rna.1796210] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/21/2023]
Abstract
In both prokaryotes and eukaryotes, the expression of a large number of genes is controlled by negative feedback, in some cases operating at the level of translation of the mRNA transcript. Of particular interest are those cases where the proteins concerned have cell-wide function in recognizing a particular codon or RNA sequence. Examples include the bacterial translation termination release factor RF2, initiation factor IF3, and eukaryote poly(A) binding protein. The regulatory loops that control their synthesis establish a negative feedback control mechanism based upon that protein's RNA sequence recognition function in translation (for example, stop codon recognition) without compromising the accurate recognition of that codon, or sequence during general, cell-wide translation. Here, the bacterial release factor RF2 and initiation factor IF3 negative feedback loops are reviewed and compared with similar negative feedback loops that regulate the levels of the eukaryote release factor, eRF1, established artificially by mutation. The control properties of such negative feedback loops are discussed as well as their evolution. The role of negative feedback to control translation factor expression is considered in the context of a growing body of evidence that both IF3 and RF2 can play a role in stimulating stalled ribosomes to abandon translation in response to amino acid starvation. Here, we make the case that negative feedback control serves primarily to limit the overexpression of these translation factors, preventing the loss of fitness resulting from an unregulated increase in the frequency of ribosome drop-off.
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Affiliation(s)
- Russell Betney
- School of Medical Sciences, Institute of Medical Sciences, University of Aberdeen, Aberdeen, AB25 2ZD, United Kingdom
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13
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Seshadri A, Dubey B, Weber MHW, Varshney U. Impact of rRNA methylations on ribosome recycling and fidelity of initiation inEscherichia coli. Mol Microbiol 2009; 72:795-808. [DOI: 10.1111/j.1365-2958.2009.06685.x] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
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14
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Qin D, Fredrick K. Control of translation initiation involves a factor-induced rearrangement of helix 44 of 16S ribosomal RNA. Mol Microbiol 2009; 71:1239-49. [PMID: 19154330 DOI: 10.1111/j.1365-2958.2009.06598.x] [Citation(s) in RCA: 34] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/30/2022]
Abstract
Initiation of translation involves recognition of the start codon by the initiator tRNA in the 30S subunit. To investigate the role of ribosomal RNA (rRNA) in this process, we isolated a number of 16S rRNA mutations that increase translation from the non-canonical start codon AUC. These mutations cluster to distinct regions that overlap remarkably well with previously identified class III protection sites and implicate both IF1 and IF3 in start codon selection. Two mutations map to the 790 loop and presumably act by inhibiting IF3 binding. Another cluster of mutations surrounds the conserved A1413(o)G1487 base pair of helix 44 in a region known to be distorted by IF1 and IF3. Site-directed mutagenesis in this region confirmed that this factor-induced rearrangement of helix 44 helps regulate initiation fidelity. A third cluster of mutations maps to the neck of the 30S subunit, suggesting that the dynamics of the head domain influences translation initiation. In addition to identifying mutations that decrease fidelity, we found that many P-site mutations increase the stringency of start codon selection. These data provide evidence that the interaction between the initiator tRNA and the 30S P site is tuned to balance efficiency and accuracy during initiation.
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Affiliation(s)
- Daoming Qin
- Ohio State Biochemistry Program, The OhioState University, OH 43210, USA
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15
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Singh NS, Ahmad R, Sangeetha R, Varshney U. Recycling of Ribosomal Complexes Stalled at the Step of Elongation in Escherichia coli. J Mol Biol 2008; 380:451-64. [DOI: 10.1016/j.jmb.2008.05.033] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/18/2008] [Revised: 05/11/2008] [Accepted: 05/15/2008] [Indexed: 10/22/2022]
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16
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Yoo JH, RajBhandary UL. Requirements for translation re-initiation in Escherichia coli: roles of initiator tRNA and initiation factors IF2 and IF3. Mol Microbiol 2008; 67:1012-26. [PMID: 18221266 PMCID: PMC2268962 DOI: 10.1111/j.1365-2958.2008.06104.x] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
Despite its importance in post-transcriptional regulation of polycistronic operons in Escherichia coli, little is known about the mechanism of translation re-initiation, which occurs when the same ribosome used to translate an upstream open reading frame (ORF) also translates a downstream ORF. To investigate translation re-initiation in Escherichia coli, we constructed a di-cistronic reporter in which a firefly luciferase gene was linked to a chloramphenicol acetyltransferase gene using a segment of the translationally coupled geneV–geneVII intercistronic region from M13 phage. With this reporter and mutant initiator tRNAs, we show that two of the unique properties of E. coli initiator tRNA – formylation of the amino acid attached to the tRNA and binding of the tRNA to the ribosomal P-site – are as important for re-initiation as for de novo initiation. Overexpression of IF2 or increasing the affinity of mutant initiator tRNA for IF2 enhanced re-initiation efficiency, suggesting that IF2 is required for efficient re-initiation. In contrast, overexpression of IF3 led to a marked decrease in re-initiation efficiency, suggesting that a 30S ribosome and not a 70S ribosome is used for translation re-initiation. Strikingly, overexpression of IF3 also blocked E. coli from acting as a host for propagation of M13 phage.
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Affiliation(s)
- Jae-Ho Yoo
- Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
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17
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Qin D, Abdi NM, Fredrick K. Characterization of 16S rRNA mutations that decrease the fidelity of translation initiation. RNA (NEW YORK, N.Y.) 2007; 13:2348-55. [PMID: 17942743 PMCID: PMC2080605 DOI: 10.1261/rna.715307] [Citation(s) in RCA: 36] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
In bacteria, initiation of translation is kinetically controlled by factors IF1, IF2, and IF3, which work in conjunction with the 30S subunit to ensure accurate selection of the initiator tRNA (fMet-tRNA(fMet)) and the start codon. Here, we show that mutations G1338A and A790G of 16S rRNA decrease initiation fidelity in vivo and do so in distinct ways. Mutation G1338A increases the affinity of tRNA(fMet) for the 30S subunit, suggesting that G1338 normally forms a suboptimal Type II interaction with fMet-tRNA(fMet). By stabilizing fMet-tRNA(fMet) in the preinitiation complex, G1338A may partially compensate for mismatches in the codon-anti-codon helix and thereby increase spurious initiation. Unlike G1338A, A790G decreases the affinity of IF3 for the 30S subunit. This may indirectly stabilize fMet-tRNA(fMet) in the preinitiation complex and/or promote premature docking of the 50S subunit, resulting in increased levels of spurious initiation.
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Affiliation(s)
- Daoming Qin
- Ohio State Biochemistry Program, The Ohio State University, Columbus, Ohio 43210, USA
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18
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Singh NS, Das G, Seshadri A, Sangeetha R, Varshney U. Evidence for a role of initiation factor 3 in recycling of ribosomal complexes stalled on mRNAs in Escherichia coli. Nucleic Acids Res 2005; 33:5591-601. [PMID: 16199751 PMCID: PMC1240113 DOI: 10.1093/nar/gki864] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Specific interactions between ribosome recycling factor (RRF) and elongation factor-G (EFG) mediate disassembly of post-termination ribosomal complexes for new rounds of initiation. The interactions between RRF and EFG are also important in peptidyl-tRNA release from stalled pre-termination complexes. Unlike the post-termination complexes (harboring deacylated tRNA), the pre-termination complexes (harboring peptidyl-tRNA) are not recycled by RRF and EFG in vitro, suggesting participation of additional factor(s) in the process. Using a combination of biochemical and genetic approaches, we show that, (i) Inclusion of IF3 with RRF and EFG results in recycling of the pre-termination complexes; (ii) IF3 overexpression in Escherichia coli LJ14 rescues its temperature sensitive phenotype for RRF; (iii) Transduction of infC135 (which encodes a functionally compromised IF3) in E.coli LJ14 generates a ‘synthetic severe’ phenotype; (iv) The infC135 and frr1 (containing an insertion in the RRF gene promoter) alleles synergistically rescue a temperature sensitive mutation in peptidyl-tRNA hydrolase in E.coli; and (v) IF3 facilitates ribosome recycling by Thermus thermophilus RRF and E.coli EFG in vivo and in vitro. These lines of evidence clearly demonstrate the physiological importance of IF3 in the overall mechanism of ribosome recycling in E.coli.
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Affiliation(s)
| | | | | | | | - U. Varshney
- To whom correspondence should be addressed. Tel: +91 80 2293 2686; Fax: +91 80 2360 2697;
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19
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Laursen BS, Sørensen HP, Mortensen KK, Sperling-Petersen HU. Initiation of protein synthesis in bacteria. Microbiol Mol Biol Rev 2005; 69:101-23. [PMID: 15755955 PMCID: PMC1082788 DOI: 10.1128/mmbr.69.1.101-123.2005] [Citation(s) in RCA: 415] [Impact Index Per Article: 21.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
Valuable information on translation initiation is available from biochemical data and recently solved structures. We present a detailed description of current knowledge about the structure, function, and interactions of the individual components involved in bacterial translation initiation. The first section describes the ribosomal features relevant to the initiation process. Subsequent sections describe the structure, function, and interactions of the mRNA, the initiator tRNA, and the initiation factors IF1, IF2, and IF3. Finally, we provide an overview of mechanisms of regulation of the translation initiation event. Translation occurs on ribonucleoprotein complexes called ribosomes. The ribosome is composed of a large subunit and a small subunit that hold the activities of peptidyltransfer and decode the triplet code of the mRNA, respectively. Translation initiation is promoted by IF1, IF2, and IF3, which mediate base pairing of the initiator tRNA anticodon to the mRNA initiation codon located in the ribosomal P-site. The mechanism of translation initiation differs for canonical and leaderless mRNAs, since the latter is dependent on the relative level of the initiation factors. Regulation of translation occurs primarily in the initiation phase. Secondary structures at the mRNA ribosomal binding site (RBS) inhibit translation initiation. The accessibility of the RBS is regulated by temperature and binding of small metabolites, proteins, or antisense RNAs. The future challenge is to obtain atomic-resolution structures of complete initiation complexes in order to understand the mechanism of translation initiation in molecular detail.
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Affiliation(s)
- Brian Søgaard Laursen
- Department of Molecular Biology, Aarhus University, Gustav Wieds vej 10C, DK-8000 Aarhus C, Denmark
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20
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Gualerzi CO, Brandi L, Caserta E, Garofalo C, Lammi M, La Teana A, Petrelli D, Spurio R, Tomsic J, Pon CL. Initiation factors in the early events of mRNA translation in bacteria. COLD SPRING HARBOR SYMPOSIA ON QUANTITATIVE BIOLOGY 2003; 66:363-76. [PMID: 12762039 DOI: 10.1101/sqb.2001.66.363] [Citation(s) in RCA: 82] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
Affiliation(s)
- C O Gualerzi
- Laboratory of Genetics, Department of Biology, MCA University of Camerino 62032, Camerino, MC, Italy
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21
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Binns N, Masters M. Expression of the Escherichia coli pcnB gene is translationally limited using an inefficient start codon: a second chromosomal example of translation initiated at AUU. Mol Microbiol 2002; 44:1287-98. [PMID: 12068810 DOI: 10.1046/j.1365-2958.2002.02945.x] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Expression of the gene pcnB, encoding the dispensable Escherichia coli poly(A) polymerase (PAPI), which is toxic when overproduced, was investigated. Its promoter was identified and found to be moderately strong when used to express a beta-galactosidase reporter. Expression levels were not affected by increasing or decreasing PcnB concentration. Translation of pcnB was found to initiate from the non-canonical initiation codon AUU. The only other coli gene reported to use AUU as initiation codon is infC, which encodes the initiation factor IF-3. AUU, in common with other rarely used initiation codons, is discriminated against by IF-3, resulting in the aborting of most AUU-promoted initiation events. This enables AUU to form part of an autoregulatory circuit controlling IF-3 production. We show that InfC discrimination reduces PcnB production fivefold. This is the first instance of this mechanism being used to limit severely the production of a potentially toxic product.
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Affiliation(s)
- Nigel Binns
- Institute of Cell and Molecular Biology, University of Edinburgh, The King's Buildings, Mayfield Road, Edinburgh EH9 3JR, Scotland, UK
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22
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Petrelli D, LaTeana A, Garofalo C, Spurio R, Pon CL, Gualerzi CO. Translation initiation factor IF3: two domains, five functions, one mechanism? EMBO J 2001; 20:4560-9. [PMID: 11500382 PMCID: PMC125572 DOI: 10.1093/emboj/20.16.4560] [Citation(s) in RCA: 96] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Initiation factor IF3 contains two domains separated by a flexible linker. While the isolated N-domain displayed neither affinity for ribosomes nor a detectable function, the isolated C-domain, added in amounts compensating for its reduced affinity for 30S subunits, performed all activities of intact IF3, namely: (i) dissociation of 70S ribosomes; (ii) shift of 30S-bound mRNA from 'stand-by' to 'P-decoding' site; (iii) dissociation of 30S-poly(U)-NacPhe-tRNA pseudo- initiation complexes; (iv) dissociation of fMet-tRNA from initiation complexes containing mRNA with the non-canonical initiation triplet AUU (AUUmRNA); (v) stimulation of mRNA translation regardless of its start codon and inhibition of AUUmRNA translation at high IF3C/ribosome ratios. These results indicate that while IF3 performs all its functions through a C-domain-30S interaction, the N-domain function is to provide additional binding energy so that its fluctuating interaction with the 30S subunit can modulate the thermodynamic stability of the 30S-IF3 complex and IF3 recycling. The localization of IF3C far away from the decoding site and anticodon stem-loop of P-site-bound tRNA indicates that the IF3 fidelity function does not entail its direct contact with these structures.
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Affiliation(s)
- Dezemona Petrelli
- Laboratory of Genetics, Department of Biology MCA, University of Camerino, I-62032 Camerino (MC) and Institute of Biochemistry, University of Ancona, I-60131 Ancona, Italy Corresponding author e-mail
| | - Anna LaTeana
- Laboratory of Genetics, Department of Biology MCA, University of Camerino, I-62032 Camerino (MC) and Institute of Biochemistry, University of Ancona, I-60131 Ancona, Italy Corresponding author e-mail
| | - Cristiana Garofalo
- Laboratory of Genetics, Department of Biology MCA, University of Camerino, I-62032 Camerino (MC) and Institute of Biochemistry, University of Ancona, I-60131 Ancona, Italy Corresponding author e-mail
| | - Roberto Spurio
- Laboratory of Genetics, Department of Biology MCA, University of Camerino, I-62032 Camerino (MC) and Institute of Biochemistry, University of Ancona, I-60131 Ancona, Italy Corresponding author e-mail
| | - Cynthia L. Pon
- Laboratory of Genetics, Department of Biology MCA, University of Camerino, I-62032 Camerino (MC) and Institute of Biochemistry, University of Ancona, I-60131 Ancona, Italy Corresponding author e-mail
| | - Claudio O. Gualerzi
- Laboratory of Genetics, Department of Biology MCA, University of Camerino, I-62032 Camerino (MC) and Institute of Biochemistry, University of Ancona, I-60131 Ancona, Italy Corresponding author e-mail
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23
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O'Connor M, Gregory ST, Rajbhandary UL, Dahlberg AE. Altered discrimination of start codons and initiator tRNAs by mutant initiation factor 3. RNA (NEW YORK, N.Y.) 2001; 7:969-78. [PMID: 11453069 PMCID: PMC1370149 DOI: 10.1017/s1355838201010184] [Citation(s) in RCA: 37] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/22/2023]
Abstract
IF3 is essential for ensuring the fidelity of the initiation step of translation in bacterial cells. Mutations at residues R99 and R131 in the C-terminal domain of the factor have previously been shown to increase initiation from the non-canonical GUA codon. Here we show that these mutant forms of IF3 fail to discriminate against initiation from many different non-AUG codons. They also enhance the activity of mutant tRNAs carrying changes in the three consecutive G-C pairs that are conserved in the anticodon stem of initiator tRNAs. In addition, the IF3 mutants stimulate initiations from leaderless mRNAs and from internal initiation codons, in the absence of any SD-anti-SD interaction. These results indicate that IF3 ensures the accuracy of initiation by inspecting both the codon-anticodon pairing and unique features of the initiator tRNA as well as suppressing initiation from other potential start sites within the mRNA.
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Affiliation(s)
- M O'Connor
- Department of Molecular and Cellular Biology and Biochemistry, Brown University, Providence, Rhode Island 02912, USA. Michael_O'
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24
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Viswanathan M, Lovett ST. Exonuclease X of Escherichia coli. A novel 3'-5' DNase and Dnaq superfamily member involved in DNA repair. J Biol Chem 1999; 274:30094-100. [PMID: 10514496 DOI: 10.1074/jbc.274.42.30094] [Citation(s) in RCA: 58] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
DNA exonucleases are critical for DNA replication, repair, and recombination. In the bacterium Escherichia coli there are 14 DNA exonucleases including exonucleases I-IX (including the two DNA polymerase I exonucleases), RecJ exonuclease, SbcCD exonuclease, RNase T, and the exonuclease domains of DNA polymerase II and III. Here we report the discovery and characterization of a new E. coli exonuclease, exonuclease X. Exonuclease X is a member of a superfamily of proteins that have homology to the 3'-5' exonuclease proofreading subunit (DnaQ) of E. coli DNA polymerase III. We have engineered and purified a (His)(6)-exonuclease X fusion protein and characterized its activity. Exonuclease X is a potent distributive exonuclease, capable of degrading both single-stranded and duplex DNA with 3'-5' polarity. Its high affinity for single-strand DNA and its rapid catalytic rate are similar to the processive exonucleases RecJ and exonuclease I. Deletion of the exoX gene exacerbated the UV sensitivity of a strain lacking RecJ, exonuclease I, and exonuclease VII. When overexpressed, exonuclease X is capable of substituting for exonuclease I in UV repair. As we have proposed for the other single-strand DNA exonucleases, exonuclease X may facilitate recombinational repair by pre-synaptic and/or post-synaptic DNA degradation.
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Affiliation(s)
- M Viswanathan
- Department of Biology, Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02254-9110, USA
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25
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Meinnel T, Sacerdot C, Graffe M, Blanquet S, Springer M. Discrimination by Escherichia coli initiation factor IF3 against initiation on non-canonical codons relies on complementarity rules. J Mol Biol 1999; 290:825-37. [PMID: 10398584 DOI: 10.1006/jmbi.1999.2881] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Translation initiation factor IF3, one of three factors specifically required for translation initiation in Escherichia coli, inhibits initiation on any codon other than the three canonical initiation codons, AUG, GUG, or UUG. This discrimination against initiation on non-canonical codons could be due to either direct recognition of the two last bases of the codon and their cognate bases on the anticodon or to some ability to "feel" codon-anticodon complementarity. To investigate the importance of codon-anticodon complementarity in the discriminatory role of IF3, we constructed a derivative of tRNALeuthat has all the known characteristics of an initiator tRNA except the CAU anticodon. This tRNA is efficiently formylated by methionyl-tRNAfMettransformylase and charged by leucyl-tRNA synthetase irrespective of the sequence of its anticodon. These initiator tRNALeuderivatives (called tRNALI) allow initiation at all the non-canonical codons tested, provided that the complementarity between the codon and the anticodon of the initiator tRNALeuis respected. More remarkably, the discrimination by IF3, normally observed with non-canonical codons, is neutralised if a tRNALIcarrying a complementary anticodon is used for initiation. This suggests that IF3 somehow recognises codon-anticodon complementarity, at least at the second and third position of the codon, rather than some specific bases in either the codon or the anticodon.
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Affiliation(s)
- T Meinnel
- Laboratoire de Biochimie UMR7654 du CNRS, Ecole Polytechnique, Palaiseau Cedex, 91128, France
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26
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Tedin K, Moll I, Grill S, Resch A, Graschopf A, Gualerzi CO, Bläsi U. Translation initiation factor 3 antagonizes authentic start codon selection on leaderless mRNAs. Mol Microbiol 1999; 31:67-77. [PMID: 9987111 DOI: 10.1046/j.1365-2958.1999.01147.x] [Citation(s) in RCA: 74] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
In this study, we have examined the influence of initiation factors on translation initiation of leaderless mRNAs whose 5'-terminal residues are the A of the AUG initiating codon. A 1:1 ratio of initiation factors to ribosomes abolished ternary complex formation at the authentic start codon of different leaderless mRNAs. Supporting this observation, in vitro translation assays using limiting ribosome concentrations with competing leaderless lambda cl and Escherichia coli ompA mRNAs, the latter containing a canonical ribosome binding site, revealed reduced cl synthesis relative to OmpA in the presence of added initiation factors. Using in vitro toeprinting and in vitro translation assays, we show that this effect can be attributed to IF3. Moreover, in vivo studies revealed that the translational efficiency of a leaderless reporter gene is decreased with increased IF3 levels. These studies are corroborated by the observed increased translational efficiency of a leaderless reporter construct in an infC mutant strain unable to discriminate against non-standard start codons. These results suggest that, in the absence of a leader or a Shine-Dalgarno sequence, the function(s) of IF3 limits stable 30S ternary complex formation.
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Affiliation(s)
- K Tedin
- Institute of Microbiology and Genetics, Vienna Biocenter, University of Vienna, Austria
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27
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Viswanathan M, Dower KW, Lovett ST. Identification of a potent DNase activity associated with RNase T of Escherichia coli. J Biol Chem 1998; 273:35126-31. [PMID: 9857048 DOI: 10.1074/jbc.273.52.35126] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
RNase T was first identified as an enzyme responsible for end turnover of tRNA in Escherichia coli. Its activity, specific for tRNA-C-C-A, catalyzes the release of tRNA-C-C and AMP. RNase T, along with several other RNases, plays a role in maturation of several other RNA species by a similar limited nuclease activity. In previous work, we identified the gene for RNase T, rnt, as a high copy suppressor of the UV sensitivity conferred by deficiency in three single-strand DNA-specific exonucleases, RecJ, exonuclease I, and exonuclease VII. This suggested that RNase T may process DNA substrates as well. In this work, we show that purified RNase T possesses a potent 3' to 5' single-strand DNA-specific exonucleolytic activity. Its Km for single-strand DNA substrates is many orders of magnitude lower than that for tRNA, suggesting that single-strand DNA may be a natural biological substrate for RNase T. We suggest that the DNase activity of RNase T may play a role in end trimming reactions during DNA recombination and/or DNA repair.
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Affiliation(s)
- M Viswanathan
- Department of Biology and Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02254-9110, USA
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