2'-5'-Linked oligo(adenylic acid) and its analogs. A new class of inhibitors of mRNA methylation.

Most eukaryotic cellular and viral mRNAs have a blocked, methylated 5' terminus, commonly referred to as a "cap." The 5'-terminal 7-methyl-G in mRNAs is essential for their efficient translation in vitro and may also protect mRNAs against exonucleases. We have explored whether enzymes which synthesize 5' caps of mRNAs are targets for antiviral agents. We have reported earlier that the 5'-triphosphate of a broad spectrum antiviral agent, Ribavirin, and the 5'-mono- and triphosphates of 2'-5'-linked oligo(adenylic acid) (2-5A) synthesized by interferon-treated cells on exposure to double-stranded RNA or EMC virus inhibit in vitro methylation of unmethylated vaccinia RNA by a crude mRNA methylating enzyme system from vaccinia virus. We report here that although 2-5A inhibits both purified vaccinia viral and cellular mRNA (guanine-7-)-methyltransferases at micromolar concentrations, the 3'-O-methylated analogs of 2-5A, methylated in the 3'-terminal-OH or methylated at all three 3'-OH groups and with varying numbers of phosphate groups, specifically inhibited the viral enzyme at submicromolar concentrations. The inhibition is noncompetitive with respect to S-adenosylmethionine, but competitive with respect to mRNA substrate. These compounds are at least 10 times more active than 2-5A. A specific inhibitor of viral mRNA methylation heretofore has not been reported.