Functional Dissection of an Alternatively Spliced Herpesvirus Gene by Splice Site Mutagenesis

Herpesviruses have large and complex DNA genomes. The largest among the herpesviruses, those of the cytomegaloviruses, include over 170 genes. Although most herpesvirus gene products are expressed from unspliced transcripts, a substantial number of viral transcripts are spliced. Some viral transcripts are subject to alternative splicing, which leads to the expression of several proteins from a single gene. Functional analysis of individual proteins derived from an alternatively spliced gene is difficult, as deletion and nonsense mutagenesis, both common methods used in the generation of viral gene knockout mutants, affect several or all gene products at the same time. Here, we show that individual gene products of an alternatively spliced herpesvirus gene can be inactivated selectively by mutagenesis of the splice donor or acceptor site and by intron deletion or substitution mutagenesis. We used this strategy to dissect the essential M112/113 gene of murine cytomegalovirus (MCMV), which encodes the MCMV Early 1 (E1) proteins. The expression of each of the four E1 protein isoforms was inactivated individually, and the requirement for each isoform in MCMV replication was analyzed in fibroblasts, endothelial cells, and macrophages. We show that the E1 p87 isoform, but not the p33, p36, and p38 isoforms, is essential for viral replication in cell culture. Moreover, the presence of one of the two medium-size isoforms (p36 or p38) and the presence of intron 1, but not its specific sequence, are required for viral replication. This study demonstrates the usefulness of splice site mutagenesis for the functional analysis of alternatively spliced herpesvirus genes.