mRNA Guanylation Catalyzed by the S-Adenosylmethionine-dependent Guanylyltransferase of Bamboo Mosaic Virus*

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 m⁷GMP 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 m⁷GMP. The transfer rate of m⁷GMP to guanylate is greater than to adenylate by a factor of ∼3, and the K_m 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.