A Phosphonoacetate-Resistant Mutant of Herpesvirus of Turkeys2

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Abstract
A phosphonoacetate (PA)-resistant mutant of the herpesvirus of turkeys (HVT) was isolated and characterized. The mutant of HVT resistant to PA (HVT pa) replicated in duck embryo fibroblast (DEF) culture in media containing 300 μg PA/mi, whereas the replication of the wild type of HVT (HVT wt) was completely inhibited in DEF culture in media containing 100 μg PA/mi. The HVT pa was distinct from the HVT wt in plaque morphology, but was indistinguishable antigenically and showed in vitro temperature sensitivity at 41° C (37/41° C efficiency of replication was about 5). It replicated poorly in chickens and failed to provide complete protection against challenge with Marek's disease virus (MDV). The HVT pa-induced DNA polymerase had an apparent inhibition constant for PA, an apparent Inhibition constant for pyrophosphate, and an apparent Michaelis constant for dCTP about 10, 2, and 2.5 times, respectively, greater than the constants for the HVT wt-induced enzyme and was also more thermolabile.