Identification of the thymidine kinase gene of infectious laryngotracheitis virus

An an initial step in the development of a recombinant poultry infectious laryngotracheitis virus (ILTV) vaccine, we report on the identification, cloning, and sequencing of a thymidine kinase (tk) gene from a virulent U.S. field isolate of ILTV. Degenerate oligonucleotide primers for the consensus nucleotide (ATP) binding site and the nucleoside (thymidine) binding site of other herpesvirus tk genes were used in the polymerase chain reaction (PCR) to amplify a fragment of ILTV DNA. The 344-base-pair (bp) amplified fragment was cloned into plasmid pKSII and used in Southern hybridizations to locate the ILTV tk gene on a 2.4-kb HindIII fragment. Upon cloning and sequencing this fragment, a 1089-bp open reading frame was identified, which is predicted to encode a protein demonstrating 27.9% amino acid homology to the herpes simplex virus type 1 (HSV-1) thymidine kinase protein. Analysis of the sequence revealed one region of difference from that reported for the Thorne strain of ILTV. In addition, the portion of the TK protein corresponding to the nucleotide binding domain is highly conserved among the avian herpesviruses.