Thymidine kinase deletion mutants of bovine herpesvirus-1

An infectious bovine rhinotracheitis virus which fails to produce any functional TK as a result of a deletion in the tk gene produced by the process comprising:

• (1) constructing a hybrid plasmid comprising a cloning vector and a DNA fragment of IBRV containing substantially all of the IBRV tk gene;
• (2) co-transfecting, in tk.sup.+ host cells, the hybrid plasmid of step (1) with infectious DNA from a tk.sup.- IBRV mutagen-induced mutant;
• (3) selecting, in tk.sup.- host cells, for tk.sup.+ IBRB from the virus produced in step (2);
• (4) deleting DNA sequences from the hybrid plasmid of step (1) such that less than substantially all of the IBRV tk gene is present;
• (5) co-transfecting, in tk.sup.+ host cells, IBRV tk.sup.+ DNA derived from the tk.sup.+ IBRV obtained in step (3) with the resulting tk.sup.- hybrid plasmid of step (4);
• and
• (6) selecting, in tk.sup.- host cells, for tk.sup.- IBRV from the virus produced in step (5) so as to produce tk.sup.- IBRV mutants which fail to produce any functional TK as a result of a deletion in the tk gene.