Genetic and biochemical characterization of the thymidine kinase gene from herpesvirus of turkeys

The genome of herpesvirus of turkeys: comparative analysis with Marek's disease viruses

The complete coding sequence of the herpesvirus of turkeys (HVT) unique long (U(L)) region along with the internal repeat regions has been determined. This allows completion of the HVT nucleotide sequence by linkage to the sequence of the unique short (U(S)) region. The genome is approximately 160 kbp and shows extensive similarity in organization to the genomes of Marek's disease virus serotypes 1 and 2 (MDV-1, MDV-2) and other alphaherpesviruses. The HVT genome contains 75 ORFs, with three ORFs present in two copies. Sixty-seven ORFs were identified readily as homologues of other alphaherpesvirus genes. Seven of the remaining eight ORFs are homologous to genes in MDV, but are absent from other herpesviruses. These include a gene with similarity to cellular lipases. The final, HVT-unique gene is a virus homologue of the cellular NR-13 gene, the product of which belongs to the Bcl family of proteins that regulate apoptosis. No other herpesvirus sequenced to date contains a homologue of this gene. Of potential significance is the absence of a complete block of genes within the HVT internal repeat that is present in MDV-1. These include the pp38 and meq genes, which have been implicated in MDV-1-induced T-cell lymphoma. By implication, other genes present in this region of MDV-1, but missing in HVT, may play important roles in the different biological properties of the viruses.

The genome of turkey herpesvirus

Here we present the first complete genomic sequence of Marek's disease virus serotype 3 (MDV3), also known as turkey herpesvirus (HVT). The 159,160-bp genome encodes an estimated 99 putative proteins and resembles alphaherpesviruses in genomic organization and gene content. HVT is very similar to MDV1 and MDV2 within the unique long (UL) and unique short (US) genomic regions, where homologous genes share a high degree of colinearity and their proteins share a high level of amino acid identity. Within the UL region, HVT contains 57 genes with homologues found in herpes simplex virus type 1 (HSV-1), six genes with homologues found only in MDV, and two genes (HVT068 and HVT070 genes) which are unique to HVT. The HVT US region is 2.2 kb shorter than that of MDV1 (Md5 strain) due to the absence of an MDV093 (SORF4) homologue and to differences at the UL/short repeat (RS) boundary. HVT lacks a homologue of MDV087, a protein encoded at the UL/RS boundary of MDV1 (Md5), and it contains two homologues of MDV096 (glycoprotein E) in the RS. HVT RS are 1,039 bp longer than those in MDV1, and with the exception of an ICP4 gene homologue, the gene content is different from that of MDV1. Six unique genes, including a homologue of the antiapoptotic gene Bcl-2, are found in the RS. This is the first reported Bcl-2 homologue in an alphaherpesvirus. HVT long repeats (RL) are 7,407 bp shorter than those in MDV1 and do not contain homologues of MDV1 genes with functions involving virulence, oncogenicity, and immune evasion. HVT lacks homologues of MDV1 oncoprotein MEQ, CxC chemokine, oncogenicity-associated phosphoprotein pp24, and conserved domains of phosphoprotein pp38. These significant genomic differences in and adjacent to RS and RL regions likely account for the differences in host range, virulence, and oncogenicity between nonpathogenic HVT and highly pathogenic MDV1.

Sequence of the canine herpesvirus thymidine kinase gene: taxon-preferred amino acid residues in the alphaherpesviral thymidine kinases

Multiple sequence alignments of evolutionarily related proteins are finding increasing use as indicators of critical amino acid residues necessary for structural stability or involved in functional domains responsible for catalytic activities. In the past, a number of alignments have provided such information for the herpesviral thymidine kinases, for which three-dimensional structures are not yet available. We have sequenced the thymidine kinase gene of a canine herpesvirus, and with a multiple alignment have identified amino acids preferentially conserved in either of two taxons, the genera Varicellovirus and Simplexvirus, of the subfamily Alphaherpesvirinae. Since some regions of the thymidine kinases show otherwise elevated levels of substitutional tolerance, these conserved amino acids are candidates for critical residues which have become fixed through selection during the evolutionary divergence of these enzymes. Several pairs with distinctive patterns of distribution among the various viruses occur in or near highly conserved sequence motifs previously proposed to form the catalytic site, and we speculate that they may represent interacting, co-ordinately variable residues.

Cosmid cloning and restriction endonuclease mapping of the herpesvirus of turkeys (HVT) genome

The genomic libraries of the herpesvirus of turkeys (HVT) presented to date do not include the entire genome. To construct a complete genomic library, HVT DNA was partially digested with Sau3A and inserted into the double cos site vector pcos2EMBL. The PstI maps derived from the partial Sau3A library demonstrate that the left region of the genome compared to the right area is overrepresented. Similar to the libraries of the HVT genome established earlier, a defined portion of the middle genomic region, however, is not contained in the partial Sau3A library. Cloning of HVT DNA in pcos2EMBL, employing BamHI for the partial digestion step, enabled to find the recombinant cosmid cBL267 which carries the BamHI-C and -D fragment as DNA insertion. As shown by Southern blot hybridization, the BamHI-C fragment ranges in a size to close the gap in the partial Sau3A library and thus guarantees the completeness of the genomic library of HVT which consists of seven overlapping cosmid clones (cBL1, cBL328, cBL11, cBL267, cBL27, cBL33, and cBL34).

Identification of important residues within the putative nucleoside binding site of HSV-1 thymidine kinase by random sequence selection: analysis of selected mutants in vitro

Random sequence mutagenesis in conjunction with genetic complementation was used to map the function of amino acid residues within the putative nucleoside binding site of the herpes simplex virus type 1 (HSV-1) thymidine kinase (TK). Six codons of the putative nucleoside binding site of the HSV-1 tk were substituted by a duplex of extended oligonucleotides containing 20% random sequences. Approximately 260 mutants were screened for the ability to genetically complement a TK-deficient Escherichia coli. Of those screened, 32% conferred TK activity. Approximately 60% of the TK positive clones contained single amino acid changes, 23% contained double changes, and 13.4% encoded the wild-type TK amino acid sequence. A small percentage of clones, 2.4% and 1.2%, contained triple or quadruple alterations, respectively. Three residues (D162, H163, and R164) appeared to be highly conserved especially with regard to the type of residues able to substitute. Secondary screening results indicated that several of the mutants had higher affinities for acyclovir and/or 3'-azido-3'-deoxythymidine than thymidine in complementation assays. In addition, a number of clones were unable to form colonies on selection medium at elevated temperatures (42 degrees C). Eight selected mutants were subcloned into an in vitro transcription vector and the derived transcripts used to program a rabbit reticulocyte lysate cell-free translation system. Biologically active translation products were then analyzed in vitro for thymidine kinase activity, for thermal stability, and for the ability to phosphorylate selected nucleoside analogues. Two of the eight mutants had an elevated thymidine kinase activity, two were significantly thermolabile, and three exhibited enhanced efficiency in phosphorylation of nucleoside analogues.

Role of the TK+ phenotype in the stability of pigeonpox virus recombinant

Insertion of foreign DNA containing the E. coli gpt marker by homologous recombination in the pigeonpox virus (PPV) thymidine kinase (TK) gene and selection for the presence of this DNA in the viral genome produced unstable recombinants after 3 plaque purifications. We highlight the persistence of duplicated TK DNA sequences arising from single crossing over, due to the growth advantage of TK+ virus. Restoration of the TK function by coinsertion of the vaccinia virus TK gene led to stable TK+ recombinants arising from double crossing over.

Avian herpesvirus as a live viral vector for the expression of heterologous antigens

Control of Marek's disease in the poultry industry has been successfully achieved for several decades by large-scale vaccination of day-old chickens with live herpesvirus of turkeys (HVT) strains. Several features of this virus including lack of pathogenicity and long-term immune protection due to a persistent viraemic infection made us decide to use HVT as a live viral vector for the expression of foreign antigens. Potential sites for the integration of foreign DNA in the unique short region of the HVT genome were identified by the insertion of a beta-galactosidase expression cassette. Vaccination trials with recombinant virus strains indicated that the marker gene was expressed and stably maintained during animal passage. Based on an insertion site mapping in one of the open reading frames of the unique short region, a general recombination vector was designed for the integration of foreign genes into HVT. Recombinant virus-directed expression of individual antigens from Newcastle disease virus was driven by a strong promoter element derived from the lung terminal repeat sequence of Rous sarcoma virus.

Construction of a pigeonpox virus recombinant: expression of the Newcastle disease virus (NDV) fusion glycoprotein and protection of chickens against NDV challenge

A pigeonpox transfer plasmid was constructed by cloning a 2.5 kb DNA fragment containing the viral thymidine kinase (TK) gene in the psp65 plasmid. The vaccinia virus P11K promoter followed by the NDV fusion (F) gene was inserted in the TK gene. The F gene was transferred to the viral genome by homologous recombination in pigeonpox virus infected CEF cells, transfected with the recombinant plasmid. Recombinant viruses were selected with BUdR and screened for their ability to induce fusion between adjacent cells. Because of the unexpected growth advantage of the TK+ WT over the TK- recombinants, viral purification was needed to obtain stable recombinants expressing a glycosylated and cleaved F protein. Vaccination of chickens by the follicular method induced high anti-F antibody titers and good protection against challenge with the virulent Italian NDV strain. Half of the oculonasal vaccinated chickens showed anti F antibodies and also half of them were protected. Although protection seems to be correlated with antibody titers, no neutralizing antibodies were found.