title: "Pretranscriptional capping" in the biosynthesis of cytoplasmic polyhedrosis virus mRNA.
abstract: The in vitro synthesis of cytoplasmic polyhedrosis virus (CPV) mRNA was previously shown to be dependent upon the presence of the methyl donor S-adenosylmethionine (AdoMet). We now find that the competitive inhibitor of methylation, S-adenosylhomocysteine (AdoHcy), also stimulates CPV mRNA synthesis efficiently, resulting in the synthesis of viral mRNAs containing 5'-terminal GpppA and ppA, rather than m(7)GpppAm as observed with Adomet. In addition to AdoHcy, other AdoMet analogues, including S-adenosylethionine and adenosine, also stimulate CPV mRNA synthesis but to a smaller extent than does AdoHcy or AdoMet. In order to study the relationship between cap formation and mRNA synthesis, nucleoside triphosphates were replaced in the RNA-synthesizing reaction mixture (containing AdoMet) by the corresponding beta,gamma-imido analogues, which are resistant to nucleotide phosphohydrolase, an enzyme involved in cap formation. Although mRNA synthesis occurred in the presence of UMP-pNHp or GMP-pNHp, none was observed when AMP-pNHp was substituted for ATP. Because the ATP molecule that becomes the 5'-terminal nucleotide of CPV mRNA must be cleaved at the beta-gamma position during cap formation, the results suggest that, in this viral transcription system, cap formation is prerequisite to mRNA synthesis-i.e., a "pretranscriptional" event.
mesh_terms: D000266:Adenylyl Imidodiphosphate / Q000378:metabolism; D001483:Base Sequence; D006150:Guanine Nucleotides / Q000378:metabolism; D006710:Homocysteine / Q000031:analogs & derivatives*; D007304:Insect Viruses / Q000378:metabolism*; D008745:Methylation; D012333:RNA, Messenger / Q000096:biosynthesis*; D012367:RNA, Viral / Q000096:biosynthesis*; D012435:S-Adenosylhomocysteine / Q000494:pharmacology*; D012436:S-Adenosylmethionine / Q000494:pharmacology*; D014158:Transcription, Genetic
chemical_list: D006150:Guanine Nucleotides; D012333:RNA, Messenger; D012367:RNA, Viral; D006710:Homocysteine; D000266:Adenylyl Imidodiphosphate; D012436:S-Adenosylmethionine; D012435:S-Adenosylhomocysteine