Mapping and Transcriptional Analysis of the Murine Cytomegalovirus Homologue of the Human Cytomegalovirus UL103 Open Reading Frame

Murine cytomegalovirus (MCMV) has been widely used as a model for human cytomegalovirus (HCMV) infection since the two viruses share many biological and pathogenic properties. To further evaluate the similarities in the genomes of these viruses we have sought to identify MCMV genes that encode immunoreactive proteins by screening a lambda gt11 cDNA library with anti-MCMV monoclonal antibodies (MAb). Clone 430, identified on the basis of its reactivity with the MAb 6/20/1, was completely sequenced and shown to encode the MCMV homologue of the HCMV UL103 open reading frame (ORF). This ORF is encoded by a 1.1-kb transcript that is expressed at late times following infection. Sequencing of the cDNA also revealed the presence of two incomplete ORFs encoding the homologues of the HCMV UL102 and 104 ORFs. These ORFs are encoded by a 2.4-kb leaky late transcript and a 3.0-kb late transcript, respectively. Together these ORFs constitute part of a block of genes conserved in all herpesviruses, and in the MCMV and HCMV genomes this block of genes shows striking positional homology. The high degree of homology between HCMV and MCMV ORFs, at both the nucleotide and amino acid levels, plus the growing evidence of conservation in gene order between the two genomes suggests that the two viruses are very similar at the molecular level.