The genome of herpes simplex virus: structure, replication and evolution

The objectives of this paper are to discuss the structure and genetic content of the genome of herpes simplex virus type 1 (HSV-1), the nature of virus DNA replicative processes, and aspects of the evolution of the virus DNA, in particular those bearing on DNA replication. We are in the late stages of determining the complete sequence of the DNA of HSV-1, which contains about 155,000 base pairs, and thus the treatment is primarily from a viewpoint of DNA sequence and organization. The genome possesses around 75 genes, generally densely arranged and without long range ordering. Introns are present in only a few genes. Protein coding sequences have been predicted, and the functions of the proteins are being pursued by various means, including use of existing genetic and biochemical data, computer based analyses, expression of isolated genes and use of oligopeptide antisera. Many proteins are known to be virion structural components, or to have regulatory roles, or to function in synthesis of virus DNA. Many, however, still lack an assigned function. Two classes of genetic entities necessary for virus DNA replication have been characterized: cis-acting sequences, which include origins of replication and packaging signals, and genes encoding proteins involved in replication. Aside from enzymes of nucleotide metabolism, the latter include DNA polymerase, DNA binding proteins, and five species detected by genetic assays, but of presently unknown functions. Complete genome sequences are now known for the related alphaherpesvirus varicella-zoster virus and for the very distinct gammaherpesvirus Epstein-Barr virus. Comparisons between the three sequences show various homologies, and also several types of divergence and rearrangement, and so allow models to be proposed for possible events in the evolution of present day herpesvirus genomes. Another aspect of genome evolution is seen in the wide range of overall base compositions found in present day herpesvirus DNAs. Finally, certain herpesvirus genes are homologous to non-herpesvirus genes, giving a glimpse of more remote relationships.