Deletions in the genomes of pseudorabies virus vaccine strains and existence of four isomers of the genomes

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Abstract
As part of a study to identify the genes responsible for the virulence of pseudorabies virus, the genomes of independently derived vaccine strains (Bartha and Norden) were mapped by restriction enzyme analysis. The structures of these genomes have been compared with that of the genome of a laboratory strain previously mapped, of restriction fragments which had been cloned. The genome of the Bartha strain was found to be very similar to that of other pseudorabies virus strain, except that a deletion of .apprx. 2.7 .times. 106 daltons was found in the unique short (US) region. This deletion was also observed in the genome of the Norden vaccine strain, but was not observed in the genomes of any other pseudorabies virus strains that have been studied (> 20). The genome of the Norden strain differs from that of other pseudorabies with strains in several other respects as well. The most important difference is that in contrast to all other pseudorabies virus strains analyzed to date, which contain a type 2 herpesvirus DNA molecule (in which the US region only inverts itself relative to the unique long [UL] region), the genome of the Norden strain is a type 3 molecule in which both the US and the UL regions of the genome invert themselves, giving rise to 4 isomeric forms of the genome. The ability of the UL region to invert itself is probably related to the fact that a sequence normally present in all other pseudorabies virus strains at the end of the UL only is found also in inverted form at the junction of the UL and the internal inverted repeat in the Norden strain.