HeLa cell RNA (2'-O-methyladenosine-N6-)-methyltransferase specific for the capped 5'-end of messenger RNA

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.