mRNA guanylation catalyzed by the S-adenosylmethionine-dependent guanylyltransferase of bamboo mosaic virus

Abstract

The S-adenosylmethionine-dependent guanylyltransferase of bamboo mosaic virus belongs to a novel class of mRNA capping enzymes distantly conserved in Alphavirus-like superfamily. The reaction sequence of the viral enzyme has been proposed comprising steps of 1) binding of GTP and S-adenosylmethionine, 2) formation of m7GTP and S-adenosylhomocysteine, 3) formation of the covalent (Enzyme-m7GMP) intermediate, and 4) transfer of m 7GMP from the intermediate to the RNA acceptor. In this study the acceptor specificity of the viral enzyme was characterized. The results show that adenylate or guanylate with 5′-diphosphate group is an essential feature for acceptors, which can be RNA or mononucleotide, to receive m 7GMP. The transfer rate of m7GMP to guanylate is greater than to adenylate by a factor of ∼3, and the Km value for mononucleotide acceptor is ∼103-fold higher than that for RNA. The capping efficiency of the viral genomic RNA transcript depends on the length of the transcript and the formation of a putative stem-loop structure, suggesting that mRNA capping process may participate in regulating the viral gene expression. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.