This RNA methyltransferase domain [1] is found in a wide range of ssRNA viruses, including Hordei-, Tobra-, Tobamo-, Bromo-, Clostero- and Caliciviruses. This methyltransferase is involved in mRNA capping. Capping of mRNA enhances its stability. This usually occurs in the nucleus. Therefore, many viruses that replicate in the cytoplasm encode their own [1]. This is a specific guanine-7-methyltransferase domain involved in viral mRNA cap0 synthesis. Specificity for guanine 7 position is shown by NMR in [3] and in vivo role in cap synthesis [4]. Based on secondary structure prediction, the basic fold is believed to be similar to the common AdoMet-dependent methyltransferase fold [5]. A curious feature of this methyltransferase domain is that it together with flanking sequences seems to have guanylyltransferase activity coupled to the methyltransferase activity [5]. The domain is found throughout the so-called Alphavirus superfamily, (including alphaviruses and several other groups). It forms the defining, unique feature of this superfamily [2].

1: The N-terminal half of the brome mosaic virus 1a protein has RNA capping-associated activities: specificity for GTP and S- adenosylmethionine. Kong F, Sivakumaran K, Kao C; Virology 1999;259:200-210. PMID:10364504

2: Conservation of the putative methyltransferase domain: a hallmark of the 'Sindbis-like' supergroup of positive-strand RNA viruses. Rozanov MN, Koonin EV, Gorbalenya AE; J Gen Virol 1992;73:2129-2134. PMID:1645151

3: Putative RNA capping activities encoded by brome mosaic virus: methylation and covalent binding of guanylate by replicase protein 1a. Ahola T, Ahlquist P; J Virol 1999;73:10061-10069. PMID:10559320

4: Helicase and capping enzyme active site mutations in brome mosaic virus protein 1a cause defects in template recruitment, negative-strand RNA synthesis, and viral RNA capping. Ahola T, den Boon JA, Ahlquist P; J Virol 2000;74:8803-8811. PMID:10982322

5: Critical residues of Semliki Forest virus RNA capping enzyme involved in methyltransferase and guanylyltransferase-like activities. Ahola T, Laakkonen P, Vihinen H, Kaariainen L; J Virol 1997;71:392-397. PMID:8985362