Catalytic activity of vaccinia mRNA capping enzyme subunits coexpressed in Escherichia coli

RNA triphosphatase, RNA guanylyltransferase, and RNA (guanine-7)-methyltransferase activities are associated with the vaccinia virus mRNA capping enzyme, a heterodimeric protein containing polypeptides of Mr 95,000 and Mr 31,000. The genes encoding the large and small subunits (corresponding to the D1 and the D12 ORFs, respectively, of the viral genome) were coexpressed in Escherichia coli BL21 (DE3) under the control of a bacteriophage T7 promoter. Guanylyltransferase activity (assayed as the formation of a covalent enzyme-guanylate complex) was detected in soluble lysates of these bacteria. A 1000-fold purification of the guanylyltransferase was achieved by ammonium sulfate precipitation and chromatography using phosphocellulose and SP5PW columns. Partially purified guanylytransferase synthesized GpppA caps when provided with 5'-triphosphate-terminated poly(A) as a cap acceptor. In the presence of AdoMet the enzyme catalyzed concomitant cap methylation with 99% efficiency. Inclusion of S-adenosyl methionine increased both the rate and extent of RNA capping, permitting quantitative modification of RNA 5' ends. Guanylyltransferase sedimented as a single component of 6.5 S during further purification in a glycerol gradient; this S value is identical with that of the heterodimeric capping enzyme from vaccinia virions. Electrophoretic analysis showed a major polypeptide of Mr 95,000 cosedimenting with the guanylyltransferase. RNA triphosphatase activity cosedimented exactly with guanylyltransferase. Methyltransferase activity was associated with guanylyltransferase and was also present in less rapidly sedimenting fractions. The methyltransferase activity profile correlated with the presence of a Mr 31,000 polypeptide. These results indicate that the D1 and D12 gene products are together sufficient to catalyze all three enzymatic steps in cap synthesis. A model for the domain structure of this enzyme is proposed.