In capping cellular mRNAs, a covalent GMP-enzyme intermediate leads to formation of G(S')ppp(5')N at time 5' end of the RNA, which is modified by methylation catalyzed by guanine-7-methyltransferase. Here we show that isolated membranes from tobacco mosaic virus (TMV)-infected plant or insect cells expressing TMV replicase protein p126, synthesized m(7) GTP using S-adenosylmethionine (AdoMet) as the methyl donor, and catalyzed the formation of a covalent guanylate-p126 complex in the presence of AdoMet, The methyl group from AdoMet was incorporated into p126, suggesting that the complex consisted of m(7)GMP-p126, Thus, TMV and alphaviruses, despite their evolutionary distance, share the same virus-specific capping mechanism. (C) 1999 Federation of European Biochemical Societies.