title: Modification of RNA by mRNA guanylyltransferase and mRNA (guanine-7-)methyltransferase from vaccinia virions.
abstract: A purified enzyme system isolated from vaccinia virus cores has been shown to modify the 5' termini of viral mRNA and synthetic poly(A) and poly(G) to form the structures m7G(5')pppA- and m7G(5')pppG-. The enzyme system has both guanylyltransferase and methyltransferase activities. The GTP:mRNA guanylyltransferase activity incorporates GMP into the 5' terminus via a 5'-5' triphosphate bond. The properties of this reaction are: (a) of the four nucleoside triphosphates only GTP is a donor, (b) mRNA with two phosphates at the 5' terminus is an acceptor while RNA with a single 5'-terminal phosphate is not, (c) Mg2+ is required, (d) the pH optimum is 7.8, (e) PP1 is a strong inhibitor, and (f) the reverse reaction, namely the formation of GTP from PP1 and RNA containing the 5'-terminal structure G(5')pppN-, readily occurs. The S-adenosylmethionine:mRNA(guanine-7-)methyltransferase activity catalyzes the methylation of the 5'-terminal guanosine. This reaction exhibits the following characteristics: (a) mRNA with the 5'-terminal sequences G(5')pppA- and G(5')pppG- are acceptors, (b) only position 7 of the terminal guanosine is methylated; internal or conventional 5'-terminal guanosine residues are not methylated, (c) the reaction is not dependent upon GTP or divalent cations, (d) optimal activity is observed in a broad pH range around neutrality, (e) the reaction is inhibited by S-adenosylhomocysteine. Both the guanylyltransferase and methyltransferase reactions exhibit bisubstrate kinetics and proceed via a sequential mechanism. The reactions may be summarized: (see article).
mesh_terms: D002413:Cations, Divalent; D006150:Guanine Nucleotides; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008780:Methyltransferases / Q000378:metabolism*; D009713:Nucleotidyltransferases / Q000378:metabolism*; D011061:Poly A; D012333:RNA, Messenger; D012435:S-Adenosylhomocysteine / Q000494:pharmacology; D012436:S-Adenosylmethionine / Q000494:pharmacology; D013329:Structure-Activity Relationship; D014616:Vaccinia virus / Q000201:enzymology*
chemical_list: D002413:Cations, Divalent; D006150:Guanine Nucleotides; D012333:RNA, Messenger; D011061:Poly A; D012436:S-Adenosylmethionine; D012435:S-Adenosylhomocysteine; D008780:Methyltransferases; D009713:Nucleotidyltransferases; D008274:Magnesium