The population of herpesvirus saimiri (HVS) genomes extracted from extracellular virions are double-stranded, linear DNA molecules of about 160 kilobase pairs (kbp) each composed of a central segment of 110 to 112 kbp and 36% (G + C) (i.e. ‘light’ or L-DNA) linked to direct reiterations of a 1.44 kbp repeat unit of 71% (G + C) (i.e. ‘heavy’ or H-DNA) at each terminus. In this paper, we show that the population of HVS DNA molecules contains approximately equal concentrations of genomes with all possible integral numbers of complete repeat units (i.e. from > 30 to 1) at either ‘left’ or ‘right’ ends but that all molecular ends are derived by a unique cleavage at a site close to the single I restriction endonuclease site of the H-DNA repeat unit. Junctions of proximal H-DNA repeat units with L-DNA occur at, or very close to, the sequence present at the molecular ends. The transition from L- to H-DNA occurs abruptly at this site at the ‘right’ end of the L-DNA component but some rearranged restriction enzyme cleavage sites typical of H-DNA are found within the first 0.8 kbp of the L-DNA sequences at the ‘left’ H-L DNA junction. HVS appears to provide an extreme example of the general process whereby herpesvirus DNAs are matured from concatemeric intermediates by a site-specific cleavage/recombination process involving random choice between equivalent sites for the initiation of the process and with choices between alternative termination sites being limited by a headful packaging mechanism.


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