A Single Amino Acid Change in the L-Polymerase Protein of Vesicular Stomatitis Virus Completely Abolishes Viral mRNA Cap Methylation

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Abstract
The vesicular stomatitis virus (VSV) RNA polymerase synthesizes viral mRNAs with 5′-cap structures methylated at the guanine- N 7 and 2′- O -adenosine positions (7mGpppA m ). Previously, our laboratory showed that a VSV host range ( hr ) and temperature-sensitive ( ts ) mutant, hr1 , had a complete defect in mRNA cap methylation and that the wild-type L protein could complement the hr1 defect in vitro. Here, we sequenced the L, P, and N genes of mutant hr1 and found only two amino acid substitutions, both residing in the L-polymerase protein, which differentiate hr1 from its wild-type parent. These mutations (N505D and D1671V) were introduced separately and together into the L gene, and their effects on VSV in vitro transcription and in vivo chloramphenicol acetyltransferase minigenome replication were studied under conditions that are permissive and nonpermissive for hr1 . Neither L mutation significantly affected viral RNA synthesis at 34°C in permissive (BHK) and nonpermissive (HEp-2) cells, but D1671V reduced in vitro transcription and genome replication by about 50% at 40°C in both cell lines. Recombinant VSV bearing each mutation were isolated, and the hr and ts phenotypes in infected cells were the result of a single D1671V substitution in the L protein. While the mutations did not significantly affect mRNA synthesis by purified viruses, 5′-cap analyses of product mRNAs clearly demonstrated that the D1671V mutation abrogated all methyltransferase activity. Sequence analysis suggests that an aspartic acid at amino acid 1671 is a critical residue within a putative conserved S -adenosyl- l -methionine-binding domain of the L protein.