Keith J M, Ensinger M J, Moss B
J Biol Chem. 1978 Jul 25;253(14):5033-9.
A novel enzyme involved in the post-transcriptional modification of the 5'-end of mRNA has been partially purified from HeLa cells. Termed an S-adenosyl-L-methionine:RNA(2'-O-methyladenosine-N4)-methyltransferase, the enzyme specifically catalyzes the transfer of a methyl group from S-adenosylmethionine to the N6 position of a 2'-O-methyladenosine residue located within the "capped" 5'-end of mRNA. The dimethylated nucleoside, N6,2'-O-dimethyladenosine, is formed as indicated by the following reaction in which m7G(5')pppAm- represents the capped and methylated 5'-end of mRNA: AdoMet + m7G(5')pppAm- leads to AdoHcy + m7G(5')pppm6A7- Greatest activity is obtained with RNA acceptors ending in m7G(5')pppAm-; less activity is found with RNA ending in m7G(5')pppA-; and barely detectable and no activity is found with RNA ending in G(5')pppA- and ppA-, respectively. Furthermore, no activity is found with oligonucleotides such as m7G(5')pppA, m7G(5')pppAm, and m7G(5')pppAmpN indicating that a longer polymer is required. It can be concluded from the substrate specificity of the enzyme that the formation of N6,2'-O-dimethyladenosine follows the biosynthesis of molecules containing m7G(5')pppAm-N-. The molecular weight of the methyltransferase, as determined by sedimentation in sucrose gradients, is approximately 65,000.
一种参与信使核糖核酸（mRNA）5′端转录后修饰的新型酶已从人宫颈癌细胞（HeLa细胞）中部分纯化出来。该酶被称为S-腺苷-L-甲硫氨酸：RNA（2′-O-甲基腺苷-N4）-甲基转移酶，它特异性地催化甲基从S-腺苷甲硫氨酸转移至位于mRNA“加帽”5′端的2′-O-甲基腺苷残基的N6位。二甲基化核苷N6,2′-O-二甲基腺苷的形成如下反应所示，其中m7G(5′)pppAm-代表加帽且甲基化的mRNA的5′端：腺苷甲硫氨酸 + m7G(5′)pppAm- 生成腺苷高半胱氨酸 + m7G(5′)pppm6A7- 以m7G(5′)pppAm-结尾的RNA受体具有最大活性；以m7G(5′)pppA-结尾的RNA活性较低；而以G(5′)pppA-和ppA-结尾的RNA分别几乎检测不到活性和没有活性。此外，对于诸如m7G(5′)pppA、m7G(5′)pppAm和m7G(5′)pppAmpN等寡核苷酸没有活性，这表明需要更长的聚合物。从该酶的底物特异性可以得出结论，N6,2′-O-二甲基腺苷的形成遵循含有m7G(5′)pppAm-N-分子的生物合成过程。通过蔗糖梯度沉降测定，甲基转移酶的分子量约为65,000。
