title: Mechanism of mRNA capping by vaccinia virus guanylyltransferase: characterization of an enzyme--guanylate intermediate.
abstract: Vaccinia virus RNA guanylyltransferase catalyzes the transfer of GMP from GTP to the 5'-triphosphate or diphosphate terminus of RNA to generate the cap structure G(5')ppp(5')N-. The guanylylation reaction consists of a series of at least two partial reactions: (i) GTP + E in equilibrium E-pG + PPi, (ii) E-pG + (p)ppNpNpN- leads to GpppNpNpN- + E. Inthe first of these, GTP reacts with capping enzyme in the absence of an RNA acceptor to form a covalent enzyme-guanylate intermediate. The GMP is linked to the Mr 95,000 subunit of the capping enzyme via a phosphoamide bond, as judged by the acid-labile, alkali-stable nature of the bond and by the susceptibility of the linkage to cleavage by hydroxylamine at pH 4.75. The isolated enzyme-guanylate complex is able to transfer the guanylate moiety to triphosphate-terminated poly(A) to yield the 5' cap structure GpppA or to pyrophosphate to regenerate GTP. Both partial reactions of transguanylylation require a divalent cation.
mesh_terms: D006157:Guanosine Monophosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D009713:Nucleotidyltransferases / Q000378:metabolism*; D012315:RNA Caps / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism*; D012367:RNA, Viral / Q000378:metabolism*; D014616:Vaccinia virus / Q000201:enzymology*
chemical_list: D012315:RNA Caps; D012333:RNA, Messenger; D012367:RNA, Viral; D006157:Guanosine Monophosphate; D006160:Guanosine Triphosphate; D009713:Nucleotidyltransferases; C020842:guanylyltransferase