Molecular investigation of the 7.2 kb RNA of murine cytomegalovirus

HCMV encodes a stable 5 kb RNA of unknown function that is conserved across cytomegalovirus species. In vivo studies of the MCMV orthologue, a 7.2 kb RNA, demonstrated that viruses that do not express the RNA fail to establish efficient persistent replication in the salivary glands of mice. To gain further insight into the function and properties of this conserved locus, we characterized the MCMV intron in finer detail.

We show that the 7.2 kb RNA is expressed with late kinetics during productive infection of mouse fibroblasts. The termini of the precursor RNA that is processed to produce the intron were identified and we demonstrate that the m106 open reading frame, which resides on the spliced mRNA derived from precursor processing, can be translated during infection. Mapping the 5′ end of the primary transcript revealed minimal promoter elements located upstream that contribute to transcript expression. Analysis of recombinant viruses with deletions in the putative promoter elements, however, revealed these elements exert only minor effects on intron expression and viral persistence in vivo. Low transcriptional output by the putative promoter element(s) is compensated by the long half-life of the 7.2 kb RNA of approximately 28.8 hours. Detailed analysis of viral spread prior to the establishment of persistence also showed that the intron is not likely required for efficient spread to the salivary gland, but rather enhances persistent replication in this tissue site.