The DNA of field isolates and vaccine strains of pseudorabies virus (PRV) was analysed by digestion with the restriction endonuclease HI. A number of distinct restriction profiles of the field isolates obtained from different locations within Europe were observed. As for herpes simplex virus, the variations could be classified into two types: first, alterations in the mobility of fragments due to the presence of additional sequences and/or the occurrence of deletions, a phenomenon most apparent in fragments containing part or whole of the repeat sequence of PRV DNA; second, generation of differently sized fragments due to loss and/or gain of restriction endonuclease cleavage sites. By analysis of several strains with HI a small number of variable cleavage sites were identified within particular regions of the unique long (U) segment of the PRV genome. Compared to wild-type PRV, the restriction fragment patterns of vaccine strains showed characteristic alterations, including the absence of bands, which were non-variable in wild-type strains, and/or the presence of new bands some of which were submolar. Some of these characteristics could be explained by a deletion in the unique short (U) region of the genome of most vaccine strains and the occurrence of closely related variants in the uncloned vaccine virus stocks.


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