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      Viral and cellular mRNA capping: Past and prospects

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      1 , 2

      Advances in Virus Research

      Published by Elsevier Inc.

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          This chapter focuses on the history of the discovery of cap and an update of research on viral and cellular-messenger RNA (mRNA) capping. Cap structures of the type m 7 GpppN(m)pN(m)p are present at the 5′ ends of nearly all eukaryotic cellular and viral mRNAs. A cap is added to cellular mRNA precursors and to the transcripts of viruses that replicate in the nucleus during the initial phases of transcription and before other processing events, including internal N 6A methylation, 3′-poly (A) addition, and exon splicing. Despite the variations on the methylation theme, the important biological consequences of a cap structure appear to correlate with the N 7-methyl on the 5′-terminal G and the two pyrophosphoryl bonds that connect m 7G in a 5′–5′ configuration to the first nucleotide of mRNA. In addition to elucidating the biochemical mechanisms of capping and the downstream effects of this 5′- modification on gene expression, the advent of gene cloning has made available an ever-increasing amount of information on the proteins responsible for producing caps and the functional effects of other cap-related interactions. Genetic approaches have demonstrated the lethal consequences of cap failure in yeasts, and complementation studies have shown the evolutionary functional conservation of capping from unicellular to metazoan organisms.

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          Most cited references 166

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          eIF4 initiation factors: effectors of mRNA recruitment to ribosomes and regulators of translation.

          Eukaryotic translation initiation factor 4F (eIF4F) is a protein complex that mediates recruitment of ribosomes to mRNA. This event is the rate-limiting step for translation under most circumstances and a primary target for translational control. Functions of the constituent proteins of eIF4F include recognition of the mRNA 5' cap structure (eIF4E), delivery of an RNA helicase to the 5' region (eIF4A), bridging of the mRNA and the ribosome (eIF4G), and circularization of the mRNA via interaction with poly(A)-binding protein (eIF4G). eIF4 activity is regulated by transcription, phosphorylation, inhibitory proteins, and proteolytic cleavage. Extracellular stimuli evoke changes in phosphorylation that influence eIF4F activity, especially through the phosphoinositide 3-kinase (PI3K) and Ras signaling pathways. Viral infection and cellular stresses also affect eIF4F function. The recent determination of the structure of eIF4E at atomic resolution has provided insight about how translation is initiated and regulated. Evidence suggests that eIF4F is also implicated in malignancy and apoptosis.
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            Exon recognition in vertebrate splicing.

             S Berget (1995)
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              A unique cap(m7GpppXm)-dependent influenza virion endonuclease cleaves capped RNAs to generate the primers that initiate viral RNA transcription.

              We propose a mechanism for the priming of influenza viral RNA transcription by capped RNAs in which specific 5'-terminal fragments are cleaved from the capped RNAs by a virion-associated endonuclease. These fragments would serve as the actual primers for the initiation of transcription by the initial incorporation by the initial incorporation of a G residue at their 3' end. We show that virions and purified viral cores contain a unique endonuclease that cleaves RNAs containing a 5' methylated cap structure (m7GpppXm) preferentially at purine residues 10 to 14 nucleotides from the cap, generating fragments with 3'-terminal hydroxyl groups. RNAs containing the 5'-terminal structure GpppG could not be cleaved to produce these specific fragments. Consistent with our proposed mechanism, those capped fragments that function as primers could be linked to a G residue in transcriptase reactions containing alpha-32P-GTP as the only ribonucleoside triphosphate. The pattern of G and C incorporation onto these primer fragments suggests that this incorporation is directed by the second and third bases at the 3' end of the virion RNA template, which has the sequence 3' UCG. Primer fragments with a 3'-terminal A residue were used more efficiently than those with a 3'-terminal G residue, indicating a preference for generating an AGC sequence in the viral mRNA complementary to the 3' end of the virion RNA. Cleavage of the RNA primer and initiation of transcription are not necessarily coupled, because a 5' fragment isolated from one reaction could be used as a primer when added to a second reaction. Uncapped ribopolymer inhibitors of viral RNA transcription inhibited the cleavage of capped RNAs.
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                Author and article information

                Journal
                Adv Virus Res
                Adv. Virus Res
                Advances in Virus Research
                Published by Elsevier Inc.
                0065-3527
                1557-8399
                7 January 2004
                2000
                7 January 2004
                : 55
                : 135-184
                Affiliations
                [a ]AGENE Research Institute, Kamakura, 247-0063 Japan
                [b ]Center for Advanced Biotechnology and Medicine Piscataway, New Jersey 08854, USA
                Article
                S0065-3527(00)55003-9
                10.1016/S0065-3527(00)55003-9
                7131690
                11050942
                Copyright © 2000 Published by Elsevier Inc.

                Since January 2020 Elsevier has created a COVID-19 resource centre with free information in English and Mandarin on the novel coronavirus COVID-19. The COVID-19 resource centre is hosted on Elsevier Connect, the company's public news and information website. Elsevier hereby grants permission to make all its COVID-19-related research that is available on the COVID-19 resource centre - including this research content - immediately available in PubMed Central and other publicly funded repositories, such as the WHO COVID database with rights for unrestricted research re-use and analyses in any form or by any means with acknowledgement of the original source. These permissions are granted for free by Elsevier for as long as the COVID-19 resource centre remains active.

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