Abstract
mRNA guanylyltransferase has been extensively purified from calf thymus. A GTP-binding assay was used based on the observations by Shuman and Hurwitz (1981) and Venkatesan and Moss (1982) that vaccinia virus and HeLa cell mRNA guanylyltransferases bind the GMP moiety from GTP in the absence of an acceptor RNA. The mol. wt. of the purified enzyme from calf thymus, estimated by polyacrylamide gel electrophoresis in the presence of SDS, is 65 000. The major protein in the purified enzyme fraction comigrates with the peptide labelled with GMP. Based on scans of silver-stained polyacrylamide gels, mRNA guanylyltransferase constitutes greater than 50% of the protein in these fractions. The enzyme catalyzed the guanylylation at the 5' end of poly(A) with a mixture of diphosphate and triphosphate ends. No evidence was obtained for a direct interaction between mRNA guanylyltransferase and RNA polymerase B (II).