Urinary & Reproductive Infectious Disease: Genome Information

Genome Characteristics:
Herpes simplex virus type 2 has a genome of double stranded DNA which has been reported to be about 84 to 100 Mdaltons. HSV-2 genomes range in length of 154, 746 bp with base composition of 70.4% G+C content and contain 78 genes.

Diagram 1
MicrobiologyBytes, 2007: Genome.

Diagram 2
Nina Thayer, 2001: HSV-2 Genome and Proteome Properties.

The genome has a unique long (UL) and a unique short (US) region, bounded by inverted repeats (red boxes in the Diagram 1), named LTRa, LTRb and LTRc, the arrangement is shown in Diagram 2. The number of copies of the LTRa repeat may vary. The repeats allow rearrangements of the unique regions and Herpesvirus genomes exist as a mixture of 4 isomers.

Because replication takes place inside the nucleus, HSV-2 can use both the host's transcription machinery and DNA repair enzymes to support a large genome with complex arrays of genes. Herpesvirus genes, like the genes of their eukaryotic hosts, are not arranged in operons and in most cases have individual promoters. However, unlike eukaryotic genes, very few herpesvirus genes are spliced. HSV-2 genomes contain lengthy terminal repeats both direct and inverted. There are six terminal repeat arrangements and understanding how these repeats function in viral success is an interesting part of current research.

Genomic Function:
The genes are characterized as either essential or dispensable for growth in cell culture.
* Essential genes regulate transcription and are needed to construct the virion.
* Dispensable genes for the most part (50% of 78 genes) function to enhance the cellular environment for virus production, to defend the virus from the host immune system and to promote cell to cell spread. The large numbers of dispensable genes are in reality required for a productive in vivo infection. It is only in the restricted environment of laboratory cell cultures that they are dispensable.