Fig. 20-1 Herpesviridae. Negatively stained preparations of the prototype herpesvirus, herpes simplex virus type 1 (A) Enveloped particles. (B) Icosahedral capsids with 162 rapsomers Bar, 100 nm. (Courtesy Dr. E. L. Palmer.)
The virion contains over 30 proteins, of which about 6 are present in the nucleocapsid, 10-20 in the tegument, and 10 in the envelope; a smaller number are associated with the DNA in the core. The envelope proteins are mainly glycoproteins, most but not all of which project as peplomers. Antigenic relationships are complex There are some shared antigens within the family, but different species have distinct envelope glycoproteins.
The herpesvirus genome consists of a linear dsDNA molecule which is infectious under appropriate experimental conditions. There is a remarkable degree of variation in the composition, size, and structure of herpesvirus DNAs. Physical maps based on ordering of restriction endonuclease fragments of viral DNA (restriction maps) are useful for epidemiologic analysis. The genomes of most of the human herpesviruses have been sequenced, an impressive achievement since these, together with those of the poxviruses, are by far the largest human viral genomes, encoding from about 70 to around 200 proteins. The genomes of the alphaherpesviruses appear to be colinear, that is, the presence and order of the individual genes are similar. Herpesviruses genomes display some unusual features (Fig. 20-2). Reiterated DNA sequences generally occur at both ends and in some viruses also internally, dividing the genome into two unique sequences, designated large (UL) and small (Us). When these reiterated sequences are inverted in their orientation, the unique L and S components can invert, relative to one another, during replication, giving rise to two or four different isomers of the genome, present in equimolar proportions. Further, intragenomic and intergenomic recom-binational events can alter the number of any particular reiterated sequence, creating polymorphism.
TRl u IRlIRrUsTRs
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