THE MAASTRICHT STRAIN AND ENGLAND STRAIN OF RAT CYTOMEGALOVIRUS REPRESENT DIFFERENT BETAHERPESVIRUS SPECIES RATHER THAN STRAINS

The major immediate early (MIE) locus of the Maastricht strain of rat cytomegalovirus (RCMV) was found to comprise five exons of which the f irst is noncoding. The first three exons are spliced to either exon 4, generating IE1, or exon 5, generating IE2. An additional splicing eve nt unique to RCMV (Maastricht) was identified in exon 5, resulting in a 466-bp deletion. IE1 transcripts were detected exclusively during th e IE phase of infection in vitro, whereas IE2 transcripts were detecte d during both the IE and late phase of infection. The similarities bet ween amino acid sequences derived from the MIE gene of RCMV (Maastrich t) and murine cytomegalovirus are low (22 and 37% for IE1 and IE2, res pectively). Surprisingly, the similarities between the MIE proteins of RCMV (Maastricht) and the England strain of RCMV are also low (23 and 32% for IE1 and IE2, respectively). This suggests that these RCMV str ains represent different betaherpesvirus species rather than strains. This is underscored by the difference between both viruses in genome s ize as well as growth characteristics. The existence of two different RCMV-like species might have important implications for the use of the se viruses as models for human cytomegalovirus. (C) 1998 Academic Pres s.