Introduction

Despite the evolutionary and biological divergence represented by the nine human herpesviruses that have been classified into three broad subgroups, a large number of herpesvirus-common (core) gene products are evolutionarily conserved (Table 4.1 see chapter 2). These appear to carry out functions upon which every herpesvirus relies because all exhibit a common virion structure, a core genome replication process, and similar entry and egress pathways. These herpesvirus common functions are most often recognized through deduced protein sequence similarity that extends throughout alpha-, beta-, and gammaherpesviruses subfamilies infecting mammals, reptiles and birds (see Chapter 2, Table 2.2). These herpesviruses exhibit conservation that suggests a shared common ancestor at least 50 million years ago. Other evolutionarily distant herpesviruses infecting fish, amphibians, and invertebrates share less similarity with these better-studied herpesviruses, suggesting a common evolutionary origin dating back over 150 million years. In the more distant relatives, a common virion structure, genome organization and similarity across a small subset herpesvirus-common gene products provide the evidence of a common origin.

A few herpesvirus-common gene products have been recognized via a common enzymatic or binding activity long before any systematic genome sequence analysis became available. The homologous function of envelope glycoprotein B, DNA polymerase, alkaline exonuclease and single strand DNA binding protein, to give a few examples, emerged from biochemical studies in a number of herpesvirus systems.