We have characterized the gene encoding the murine cytomegalovirus (MCMV) homologue of the human cytomegalovirus (HCMV) UL100 open reading frame (ORF) that encodes the HCMV glycoprotein M (gM) molecule. It was identified based on its collinearity with MCMV homologues of the HCMV UL99, UL102, UL103 and UL104 ORFs which lie in the dIII G fragment of the K181 strain of MCMV. Sequencing of a 2·3 kb RI-HI subfragment of the RI G fragment adjacent to the RI A fragment revealed the presence of the complete MCMV gM ORF and two incomplete ORFs, which corresponded to homologues of HCMV UL99 and UL102. The MCMV gM ORF consists of 1059 nucleotides and is expressed as a 1·2 kb transcript at late times post-infection. To precisely characterize the gM transcript, the 5′ and 3′ ends were mapped. It was found that the transcript initiates at nucleotides 740 or 745, and that the site of polyadenylation at nucleotide 1961 occurs downstream of the second potential polyadenylation signal located at nucleotide 1934. Based on these findings the MCMV gM is predicted to consist of 353 residues and when compared with HCMV gM has a 47% level of identity. Of great interest is the finding that the MCMV gM amino acid sequence is completely conserved among six isolates of MCMV that had been shown to exhibit considerable variation both in the MCMV glycoprotein B and the immediate-early 1 gene-encoded pp89 molecule. Thus, this glycoprotein appears to be antigenically conserved.


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