MESSENGER RNA(NUCLEOSIDE-2'-)-METHYLTRANSFERASE FROM VACCINIA VIRUS - CHARACTERISTICS AND SUBSTRATE-SPECIFICITY

  • 1 January 1978
    • journal article
    • research article
    • Vol. 253  (21) , 7698-7702
Abstract
An mRNA(nucleoside-2''-)-methyltransferase purified from vaccinia virus methylated the penultimate nucleoside of RNA ending in m7G(5'')pppN-. By contrast, RNA ending in pN-, ppN- or even G(5'')pppN- are not methyl acceptors. This specificity indicates that 2''-O-methylation is the final step in the formation of the m7G(5'')pppNm- cap structure. Both adenosine and guanosine are methylated, in accordance with the presence of these nucleosides in the penultimate position of vaccinia virus mRNA. Studies with homopolyribonucleotides containing m7G(5'')pppN ends indicated that poly(A) and poly(I) were the best methyl acceptors while significant but much less activity was obtained with poly(G), poly(U) and poly(C). Simple dinucleotides of the type m7G(5'')pppN are poor substrates and do not complete with capped RNA. Additional studies indicate that the methyltransferase has a pH optimum of 7.5, does not require divalent cations, is inhibited by S-adenosylhomocysteine, has a Km of 2.0 .mu.M for S-adenosylmethionine, a Km of about 5 nM for brome mosaic virus RNA, and kinetics consistent with a random bireactant mechanism.