The deoxyribonucleic acid (DNA) of pseudorabies virus (PRV) was examined by restriction endonuclease analysis before and after various treatments: namely 1. plaque purification of stock virus, 2. serial passage of virus in cell culture at high (H) and low (L) multiplicity of infection (MOI), and 3. serial passage of virus in swine. The objective of the study was to determine if such treatments would either select or induce virus populations with a different predominant number or location of enzyme cleavage sites. Heterogeneity of the DNA of stock virus was revealed by differences among restriction patterns of 10 plaque-purified populations. All of these populations were then relatively stable through an additional 9 plaque purifications and passages in cell culture. Without plaque purification, heterogeneity was not evident. The predominant restriction pattern of stock virus appeared unaltered by 10 serial passages in cell culture at either HMOI or LMOI, except for the appearance in the HMOI series of several minor bands. Conversely, 10 serial passages of stock virus in swine resulted in either selection or induced changes or both. Most differences were slightly altered migration rates of relatively small (≤5 Kbp) fragments. However, after 10 passages in swine, there were also changes in the migration rates of 2 large fragments (>10 Kbp) of the viral genome.
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