Rat cytomegalovirus R89 is a highly conserved gene which expresses a spliced transcript

In all sequenced herpesvirus genomes, a homolog of the herpes simplex virus type 1 UL15 gene has been identified. This gene encodes a protein that is involved in viral genome maturation. Although transcription of the alphaherpesvirus UL15 gene has been analyzed in detail, not much is known about the expression of its betaherpesvirus homologs. We therefore set out to characterize transcription of the rat cytomegalovirus counterpart of UL15, R89. Here we report that R89 consists of two exons separated by a 4.7-kb intron. The spliced R89 transcript, which is expressed at late times postinfection (p.i.), has the capacity to encode a protein of 670 amino acids with a calculated molecular mass of 77.1 kDa. The predicted amino acid sequence of this protein is highly similar to that of the proteins predicted to be encoded by the human cytomegalovirus UL89 and murine cytomegalovirus M89 genes (64.3 and 84.5% overall identity, respectively). The region between R89 exon 1 and exon 2 was found to contain five additional genes, r90, R91, R92, R93 and R94, the latter two of which are conserved among all herpesviruses. We show that these genes are transcribed in a highly complex fashion, resulting in numerous mono- and polycistronic mRNAs.