| Abstract: | Characterization of the donor and acceptor specificities of mRNA guanylyltransferase and mRNA (guanine-7-)-methyltransferase isolated from vaccinia virus cores has enabled us to discriminate between alternative reaction sequences leading to the formation of the 5'-terminal m7G(5')pppN-structure. The mRNA guanylyltransferase catalyzes the transfer of a residue of GMP from GTP to acceptors which possess a 5'-terminal diphosphate. A diphosphate-terminated polyribonucleotide is preferred to a mononucleoside diphosphate as an acceptor suggesting that the guanylyltransferase reaction occurs after initiation of RNA synthesis. Although all of the homopolyribonucleotides tested (pp(A)n, pp(G)n, pp(I)n, pp(U)n, and pp(C)n) are acceptors for the mRNA guanylyltransferase indicating lack of strict sequence specificity, those containing purines are preferred. Only GTP and dGTP are donors in the reaction; 7-methylguanosine (m7G) triphosphate specifically is not a donor indicating that guanylylation must precede guanine-7-methylation. The preferred acceptor of the mRNA (guanine-7-)-methyltransferase is the product of the guanylyltransferase reaction, a polyribonucleotide with the 5'-terminal sequence G(5')pppN-. The enzyme can also catalyze, but less efficiently methylation of the following: dinucleoside triphosphates with the structure G(5')pppN, GTP, dGTP, ITP, GDP, GMP, and guanosine. The enzyme will not catalyze the transfer of methyl groups to ATP, XTP, CTP, UTP, or to guanosine-containing compounds with phosphate groups in either positions 2' or 3' or in 3'-5' phosphodiester linkages. The latter specificity provides an explanation for the absence of internal 7-methylguanosine in mRNA. In the presence of PPi, the mRNA guanylyltransferase catalyzes the pyrophosphorolysis of the dinucleoside triphosphate G(5')pppA, but not of m7G(5')pppA. Since PPi is generated in the process of RNA chain elongation, stabilization of the 5'-terminal sequences of mRNA is afforded by guanine-7-methylation. |
