Five thymidine kinase (TK)-deficient mutants (B1 to B5) of bovine herpesvirus type 1 (BHV-1) were isolated by selection for resistance to the nucleoside analogue bromovinyldeoxyuridine. The genetic lesion in mutant B1 was localized in a 2.7 kb I-I subfragment (fTK2.7) which maps between 0.456 and 0.475 within the dIII A fragment of the BHV-1 genome. The genes from wild-type and the TK mutants B1 to B5 were cloned and sequenced using eight unique synthetic primers designed from a published sequence. The BHV-1 gene sequence for the strain 6660 contained some differences compared with that published previously for strain LA. Alignment of the predicted amino acid sequence of the BHV-1 TK polypeptide with different herpesvirus TKs revealed five strongly conserved regions and also identified putative functional relationships with other enzymes. Several interesting features were apparent in the gene sequences from the TK mutants. The TK mutant B1 was a typical frameshift and chain termination mutant due to the deletion of a single base. The gene sequence of mutant B2 revealed the deletion of three bases resulting in the loss of valine at amino acid residue 174 of the TK polypeptide. The genes of mutants B3 to B5 contained an identical change of a single base addition resulting in frameshift and premature chain termination. In contrast to wild-type BHV-1, the TK-defective mutants were incapable of adsorbing TK-neutralizing antibodies from serum.


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