Identification and DNA sequence analysis of the Marek's disease virus serotype 2 genes homologous to the thymidine kinase and UL24 genes of herpes simplex virus type 1

The thymidine kinase (TK) gene has been used as a safe and convenient locus for expression of heterologous proteins in some alphaherpesviruses including herpesvirus of turkeys (HVT) antigenically related to Marek's disease virus (MDV) serotypes 1 (MDV1) and 2 (MDV2) in MDV2 strain HPRS 24 genome, genes equivalent to the TK and UL24 homologues of herpes simplex virus type 1 were identified and sequenced. The MDV2 UL24 gene overlaps the 5' end of the TK gene in a head-to-head orientation. The predicted region encoding for the MDV2 TK gene is 1056 nucleotides, corresponding to a polypeptide of 352 amino acids in length. Putative nucleotide- and thymidine-binding sites were identified within the predicted amino acid sequence. The predicted region encoding for the UL24 gene is 948 nucleotides, corresponding to a polypeptide of 316 amino acids in length. By northern blot analyses using MDV2 TK- and UL24-specific DNA probes, four transcripts of approximately 7.8, 5.0, 3.5, and 1.1 kb for the TK gene, and a transcript of 3.8 kb for the UL24 gene were detected in MDV2-infected cells. Alignment of the amino acid sequence of MDV2 TK homologue with those published for TK homologues of other MDV serotypes showed 73.9% (MDV1 vs. MDV2), 58.2% (MDV1 vs. HVT), and 56.8% (MDV2 vs. HVT) identities. Comparison to other alphaherpesvirus TK homologues revealed amino acid sequence homologies varying from 34.5% to 27.8%. The putative MDV2 UL24 homologous protein had identity with the well conserved five motifs among alphaherpesviruses.