YEAST MESSENGER-RNA CAP METHYLTRANSFERASE IS A 50-KILODALTON PROTEIN ENCODED BY AN ESSENTIAL GENE

RNA (guanine-7-)methyltransferase, the enzyme responsible for methylat ing the 5' cap structure of eukaryotic mRNA, was isolated from extract s of Saccharomyces cerevisiae. The yeast enzyme catalyzed methyl group transfer from S-adenosyl-L-methionine to the guanosine base of capped , unmethylated poly(A). Cap methylation was stimulated by low concentr ations of salt and was inhibited by S-adenosyl-L-homocysteine, a presu mptive product of the reaction, but not by S-adenosyl-D-homocysteine. The methyltransferase sedimented in a glycerol gradient as a single di screte component of 3.2S. A likely candidate for the gene encoding yea st cap methyltransferase was singled out on phylogenetic grounds. The ABD1 gene, located on yeast chromosome II, encodes a 436-amino-acid (5 0-kDa) polypeptide that displays regional similarity to the catalytic domain of the vaccinia virus cap methyltransferase. That the ABD1 gene product is indeed RNA (guanine-7-)methyltransferase was established b y expressing the ABD1 protein in bacteria, purifying the protein to ho mogeneity, and characterizing the cap methyltransferase activity intri nsic to recombinant ABD1. The physical and biochemical properties of r ecombinant ABD1 methyltransferase were indistinguishable from those of the cap methyltransferase isolated and partially purified from whole cell yeast extracts. Our finding that the ABD1 gene is required for ye ast growth provides the first genetic evidence that a cap methyltransf erase (and, by inference, the cap methyl group) plays an essential rol e in cellular function in vivo.