Introduction
The two major lineages in the Betaherpesvirinae are the cytomegaloviruses (the Cytomegalovirus and Muromegalovirus genera, plus a number of other viruses whose taxonomy is only partially defined) and the Roseolovirus genus (see Chapter 1). The best characterized members of these lineages are HCMV (the prototype of the subfamily) and HHV-6, respectively. Cytomegaloviruses are present in a wide range of mammalian species, and have been termed “salivary gland viruses” because of their ease of isolation from explanted tissue. An earlier divergence of the Betaherpesvirinae may be represented by a herpesvirus of elephants (Richman et al., 1999; Ehlers et al., 2001). This chapter starts by describing the genome structures of Betaherpesvirinae, then examines the genetic content of HCMV and HHV-6, and finally focuses on the virion structure of HCMV.
Genome structures
The genomes of viruses in the Roseolovirus genus are significantly smaller, at 145–162 kbp, than those of other Betaherpesvirinae, at 196–241 kbp. Indeed, HCMV has the largest genome among the human herpesviruses, and thus far its closest relative, CCMV, has the largest genome of all sequenced herpesviruses. It seems likely that the ancestor of the Betaherpesvirinae had a genome consisting of a unique region flanked by a direct repeat (the class A genome described in Chapter 2), since this structure is characteristic of most extant members of the subfamily. Earlier studies ruled out the presence of large repeats in the genomes of MCMV (Ebeling et al., 1983a; Mercer et al., 1983; Marks and Spector, 1984), THV (Koch et al., 1985) and RCMV (Meijer et al., 1986).