Martin S A, Moss B
J Biol Chem. 1976 Dec 10;251(23):7313-21.
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.
对从痘苗病毒核心分离出的mRNA鸟苷酸转移酶和mRNA（鸟嘌呤-7-）-甲基转移酶的供体和受体特异性进行表征，使我们能够区分导致形成5'-末端m7G(5')pppN结构的不同反应序列。mRNA鸟苷酸转移酶催化GMP残基从GTP转移到具有5'-末端二磷酸的受体上。作为受体，二磷酸末端的多聚核糖核苷酸比单核苷二磷酸更受青睐，这表明鸟苷酸转移酶反应发生在RNA合成起始之后。尽管所有测试的同聚多聚核糖核苷酸（pp(A)n、pp(G)n、pp(I)n、pp(U)n和pp(C)n）都是mRNA鸟苷酸转移酶的受体，表明缺乏严格的序列特异性，但含嘌呤的那些更受青睐。反应中只有GTP和dGTP是供体；7-甲基鸟苷三磷酸（m7G）特别不是供体，这表明鸟苷酸化必须先于鸟嘌呤-7-甲基化。mRNA（鸟嘌呤-7-）-甲基转移酶的首选受体是鸟苷酸转移酶反应的产物，即具有5'-末端序列G(5')pppN-的多聚核糖核苷酸。该酶还可以催化，但效率较低的是以下物质的甲基化：具有G(5')pppN结构的二核苷三磷酸、GTP、dGTP、ITP、GDP、GMP和鸟苷。该酶不会催化甲基转移到ATP、XTP、CTP、UTP或2'或3'位带有磷酸基团或3'-5'磷酸二酯键的含鸟苷化合物上。后一种特异性解释了mRNA中不存在内部7-甲基鸟苷的原因。在PPi存在的情况下，mRNA鸟苷酸转移酶催化二核苷三磷酸G(5')pppA的焦磷酸解，但不催化m7G(5')pppA的焦磷酸解。由于PPi在RNA链延伸过程中产生，鸟嘌呤-7-甲基化可稳定mRNA的5'-末端序列。
