The glycoprotein D (US6) homolog is not essential for oncogenicity or horizontal transmission of Marek's disease virus

Latest Insights into Unique Open Reading Frames Encoded by Unique Long (UL) and Short (US) Regions of Marek's Disease Virus

Marek’s disease virus (MDV) is an oncogenic avian alphaherpesvirus whose genome consists of unique long (UL) and short (US) regions that are flanked by inverted repeat regions. More than 100 open reading frames (ORFs) have been annotated in the MDV genome, and are involved in various aspects of MDV biology and pathogenesis. Within UL and US regions of MDV, there are several unique ORFs, some of which have recently been shown to be important for MDV replication and pathogenesis. In this review, we will summarize the current knowledge on these ORFs and compare their location in different MDV strains.

Identification of non-essential loci within the Meleagrid herpesvirus 1 genome

Background

Meleagrid herpesvirus 1 (MeHV-1) infectious bacterial artificial chromosomes (iBACs) are ideal vectors for the development of recombinant vaccines for the poultry industry. However, the full potential of iBACS as vectors can only be realised after thorough genetic characterisation, including identification of those genetic locations that are non-essential for virus replication. Generally, transposition has proven to be a highly effective strategy for rapid and efficient mutagenesis of iBAC clones. The current study describes the characterisation of 34 MeHV-1 mutants containing transposon insertions within the pMeHV1-C18 iBAC genome.

Methods

Tn5 and MuA transposition methods were used to generate a library of 76 MeHV-1 insertion mutants. The capacity of each mutant to facilitate the recovery of infectious MeHV-1 was determined by the transfection of clone DNA into chicken embryo fibroblasts.

Results

Attempts to recover infectious virus from the modified clones identified 14 genetic locations that were essential for MeHV-1 replication in cell culture. Infectious MeHV-1 was recovered from the remaining 14 intragenic insertion mutants and six intergenic insertion mutants, suggesting that the respective insertion locations are non-essential for MeHV-1 replication in cell culture.

Conclusions

The essential and non-essential designations for those MeHV-1 genes characterised in this study were generally in agreement with previous reports for other herpesviruses homologues. However, the requirement for the mardivirus-specific genes LORF4A and LORF5 are reported for the first time. These findings will help direct future work on the development of recombinant poultry vaccines using MeHV-1 as a vector by identifying potential transgene insertion sites within the viral genome.

Electronic supplementary material

The online version of this article (doi10.1186/s12985-015-0362-9) contains supplementary material, which is available to authorized users.

B Virus (Macacine herpesvirus 1) Glycoprotein D Is Functional but Dispensable for Virus Entry into Macaque and Human Skin Cells

Glycoprotein D (gD) plays an essential role in cell entry of many simplexviruses. B virus (Macacine herpesvirus 1) is closely related to herpes simplex virus 1 (HSV-1) and encodes gD, which shares more than 70% amino acid similarity with HSV-1 gD. Previously, we have demonstrated that B virus gD polyclonal antibodies were unable to neutralize B virus infectivity on epithelial cell lines, suggesting gD is not required for B virus entry into these cells. In the present study, we confirmed this finding by producing a B virus mutant, BV-ΔgDZ, in which the gD gene was replaced with a lacZ expression cassette. Recombinant plaques were selected on complementing VD60 cells expressing HSV-1 gD. Virions lacking gD were produced in Vero cells infected with BV-ΔgDZ. In contrast to HSV-1, B virus lacking gD was able to infect and form plaques on noncomplementing cell lines, including Vero, HEp-2, LLC-MK2, primary human and macaque dermal fibroblasts, and U373 human glioblastoma cells. The gD-negative BV-ΔgDZ also failed to enter entry-resistant murine B78H1 cells bearing a single gD receptor, human nectin-1, but gained the ability to enter when phenotypically supplemented with HSV-1 gD. Cell attachment and penetration rates, as well as the replication characteristics of BV-ΔgDZ in Vero cells, were almost identical to those of wild-type (wt) B virus. These observations indicate that B virus can utilize gD-independent cell entry and transmission mechanisms, in addition to generally used gD-dependent mechanisms.

IMPORTANCE

B virus is the only known simplexvirus that causes zoonotic infection, resulting in approximately 80% mortality in untreated humans or in lifelong persistence with the constant threat of reactivation in survivors. Here, we report that B virus lacking the gD envelope glycoprotein infects both human and monkey cells as efficiently as wild-type B virus. These data provide evidence for a novel mechanism(s) utilized by B virus to gain access to target cells. This mechanism is different from those used by its close relatives, HSV-1 and -2, where gD is a pivotal protein in the virus entry process. The possibility remains that unidentified receptors, specific for B virus, permit virus entry into target cells through gD-independent pathways. Understanding the molecular mechanisms of B virus entry may help in developing rational therapeutic strategies for the prevention and treatment of B virus infection in both macaques and humans.

Marek's disease virus and skin interactions

Marek's disease virus (MDV) is a highly contagious herpesvirus which induces T-cell lymphoma in the chicken. This virus is still spreading in flocks despite forty years of vaccination, with important economical losses worldwide. The feather follicles, which anchor feathers into the skin and allow their morphogenesis, are considered as the unique source of MDV excretion, causing environmental contamination and disease transmission. Epithelial cells from the feather follicles are the only known cells in which high levels of infectious mature virions have been observed by transmission electron microscopy and from which cell-free infectious virions have been purified. Finally, feathers harvested on animals and dust are today considered excellent materials to monitor vaccination, spread of pathogenic viruses, and environmental contamination. This article reviews the current knowledge on MDV-skin interactions and discusses new approaches that could solve important issues in the future.

Marek's disease virus morphogenesis

Marek's disease virus (MDV) is a highly contagious virus that induces T-lymphoma in chicken. This viral infection still circulates in poultry flocks despite the use of vaccines. With the emergence of new virulent strains in the field over time, MDV remains a serious threat to the poultry industry. More than 40 yr after MDV identification as a herpesvirus, the visualization and purification of fully enveloped infectious particles remain a challenge for biologists. The various strategies used to detect such hidden particles by electron microscopy are reviewed herein. It is now generally accepted that the production of cell-free virions only occurs in the feather follicle epithelium and is associated with viral, cellular, or both molecular determinants expressed in this tissue. This tissue is considered the only source of efficient virus shedding into the environment and therefore the origin of successful transmission in birds. In other avian tissues or permissive cell cultures, MDV replication only leads to a very low number of intracellular enveloped virions. In the absence of detectable extracellular enveloped virions in cell culture, the nature of the transmitted infectious material and its mechanisms of spread from cell to cell remain to be deciphered. An attempt is made to bring together the current knowledge on MDV morphogenesis and spread, and new approaches that could help understand MDV morphogenesis are discussed.

Use of interfering RNAs targeted against feline herpesvirus 1 glycoprotein D for inhibition of feline herpesvirus 1 infection of feline kidney cells

OBJECTIVE

To evaluate the use of RNA interference targeted against feline herpesvirus 1 (FHV-1) glycoprotein D for inhibition of FHV-1 infection of feline kidney cells.

SAMPLE POPULATION

Crandell-Rees feline kidney cells.

PROCEDURES

Crandell-Rees feline kidney cells were transfected with small interfering RNAs (siRNAs) that were designed to inhibit expression of FHV-1 glycoprotein D. The effectiveness of the treatment was determined via measurement of amounts of glycoprotein D mRNA, intracellular glycoprotein D, and glycoprotein D expressed on the surface of infected cells and comparison with appropriate control sample data.

RESULTS

2 of 6 siRNAs tested were highly effective in reducing expression (ie, knockdown) of glycoprotein D mRNA; there were 77% and 85% reductions in mRNA in treated samples, compared with findings in the control samples. The knockdown of glycoprotein D mRNA resulted in reduced glycoprotein D protein production, as evidenced by 27% and 43% decreases in expression of glycoprotein D on the surface of siRNA-treated, FHV-1-infected cells and decreased expression of the protein within infected cells, compared with control samples. Treatment with these siRNAs also resulted in inhibition of FHV-1 replication, with reductions of 84% and 77% in amounts of virus released into cell culture supernatant, compared with findings in control samples.

CONCLUSIONS AND CLINICAL RELEVANCE

2 chemically produced siRNAs that targeted the glycoprotein D gene significantly reduced FHV-1 titers in treated cells, suggesting that glycoprotein D is necessary for production of infective virions. This gene is a potential target for RNA interference as a means of inhibition of FHV-1 infection of feline cells.

Clustering of mutations within the inverted repeat regions of a serially passaged attenuated gallid herpesvirus type 2 strain

Marek's disease (MD) is the leading cause of losses in chicken production in the world. Over the past 40 years significant progress has been made in the control of MD through the use of vaccines which reduce or delay tumor formation in vaccinated flocks. However, these vaccines fail to induce an immune response that protects against infection and virus shedding. Little is known about the genetic changes that lead to attenuation and are necessary for the generation of vaccine strains. Previous research has demonstrated that serial passage of virulent strains in cell culture results in the generation of attenuated progeny. Obtaining detailed knowledge of the changes which are needed for attenuation will be important for advancing our understanding of MD biology and should facilitate the development of more potent vaccines. We have determined the complete nucleotide sequence of a bacterial artificial chromosome (BAC) construct representing the 80th passage of a very virulent plus (vv+) MD virus strain termed 584A. Pathotyping studies have indicated that this strain (584Ap80) is indeed attenuated. Bioinformatic analysis of the sequencing data has identified numerous gross genetic changes clustering in the inverted repeat regions of the genome, as well as subtle changes (single nucleotide polymorphisms or SNPs) scattered throughout the genome. Relative to the parental strain (584Ap9), insertional mutations were identified in the MD-specific genes encoding RLORF1, RLORF3, RLORF6, 23 kDa, RLORF7 (Meq), vIL8, vLip, RSORF1, and five uncharacterized novel genes. Deletions were found in four locations within the 584Ap80 genome. A large deletion (297nt) was found in the diploid genes 85.6/98.6 and a 321 nt deletion within the intergenic region between the U(L)3 and U(L)3.5 genes is predicted to create a fusion polypeptide. A single nucleotide deletion was identified within the origin of replication. Both insertions and deletions were found in the dipoid genes MDV3.4/78.3 encoding the virulence factor RLORF4. The sequencing of the attenuated strain 584Ap80 and comparison to that of the virulent parent 584A passage 9 (584Ap9) has provided a wealth of information regarding genetic changes which have occurred during the attenuation process.

Horizontal transmission of Marek's disease virus requires US2, the UL13 protein kinase, and gC

Marek's disease virus (MDV) causes a general malaise in chickens that is mostly characterized by the development of lymphoblastoid tumors in multiple organs. The use of bacterial artificial chromosomes (BACs) for cloning and manipulation of the MDV genome has facilitated characterization of specific genes and genomic regions. The development of most MDV BACs, including pRB-1B-5, derived from a very virulent MDV strain, involved replacement of the US2 gene with mini-F vector sequences. However, when reconstituted viruses based on pRB-1B were used in pathogenicity studies, it was discovered that contact chickens housed together with experimentally infected chickens did not contract Marek's disease (MD), indicating a lack of horizontal transmission. Staining of feather follicle epithelial cells in the skins of infected chickens showed that virus was present but was unable to be released and/or infect susceptible chickens. Restoration of US2 and removal of mini-F sequences within viral RB-1B did not alter this characteristic, although in vivo viremia levels were increased significantly. Sequence analyses of pRB-1B revealed that the UL13, UL44, and US6 genes encoding the UL13 serine/threonine protein kinase, glycoprotein C (gC), and gD, respectively, harbored frameshift mutations. These mutations were repaired individually, or in combination, using two-step Red mutagenesis. Reconstituted viruses were tested for replication, MD incidence, and their abilities to horizontally spread to contact chickens. The experiments clearly showed that US2, UL13, and gC in combination are essential for horizontal transmission of MDV and that none of the genes alone is able to restore this phenotype.

Marek's disease virus research in the post-sequencing era: new tools for the study of gene functions and virus-host interactions

Despite the fact that the causative agent of Marek's disease was described more than 30 years ago, and that subsequently many classical biological studies have been carried out on the Marek's disease virus (MDV), detailed analysis of its gene functions has been hampered by lack of suitable research tools. Information on the primary structure of MDV-1 and its serologically related viruses, MDV-2 and herpesvirus of turkeys, is now available. This review focuses on the introduction of the modern and highly efficient technology of bacterial artificial chromosome (BAC) cloning and mutagenesis for rapid manipulation of the MDV genome, with the aim of studying the functions of its genes and non-coding regions. Constructed MDV BACs carry the complete genome of MDV that can be multiplied in Escherichia coli and manipulated using the tools provided by bacterial genetics. The novel approach of MDV DNA mutagenesis using BAC technology will be explained by examples, and we will discuss gene functions in comparison with their counterparts in other herpesviruses. In addition, we have shown that MDV BAC DNA can be used as a polynucleotide vaccine to protect against Marek's disease, thus opening a new chapter in strategies for control of this disease.

Comparative sequence analysis of a highly oncogenic but horizontal spread-defective clone of Marek's disease virus

Marek's disease virus (MDV) is a cell-associated alphaherpesvirus that induces rapid-onset T-cell lymphomas in poultry. MDV isolates vary greatly in pathogenicity. While some of the strains such as CVI988 are non-pathogenic and are used as vaccines, others such as RB-1B are highly oncogenic. Molecular determinants associated with differences in pathogenicity are not completely understood. Comparison of the genome sequences of phenotypically different strains could help to identify molecular determinants of pathogenicity. We have previously reported the construction of bacterial artificial chromosome (BAC) clones of RB-1B from which fully infectious viruses could be reconstituted upon DNA transfection into chicken cells. MDV reconstituted from one of these clones (pRB-1B-5) showed similar in vitro and in vivo replication kinetics and oncogenicity as the parental virus. However, unlike the parental RB-1B virus, the BAC-derived virus showed inability to spread between birds. In order to identify the unique determinants for oncogenicity and the ''non-spreading phenotype'' of MDV derived from this clone, we determined the full-length sequence of pRB-1B-5. Comparative sequence analysis with the published sequences of strains such as Md5, Md11, and CVI988 identified frameshift mutations in RLORF1, protein kinase (UL13), and glycoproteins C (UL44) and D (US6). Comparison of the sequences of these genes with the parental virus indicated that the RLORF1, UL44, and US6 mutations were also present in the parental RB-1B stock of the virus. However with regard to UL13 mutation, the parental RB-1B stock appeared to be a mixture of wild type and mutant viruses, indicating that the BAC cloning has selected a mutant clone. Although further studies are needed to evaluate the role of these genes in the horizontal-spreading defective phenotype, our data clearly indicate that mutations in these genes do not affect the oncogenicity of MDV.

Direct evidence of host genome acquisition by the alphaherpesvirus Marek’s disease virus

Many herpesviruses including Marek's disease virus (MDV), a poultry alphaherpesvirus, carry homologous host genes presumably acquired during viral evolution. We have characterized one recent acquisition by MDV in considerable detail. The virulent MDV strain Md11 previously was isolated from a commercial chicken and initially propagated on duck cells. In the process of cloning the entire Md11 genome in a bacterial artificial chromosome (BAC), we obtained an infectious clone in which the entire terminal repeat short segment was replaced with a portion of the duck genome that corresponds to chicken chromosome 19. This sequence is not predicted to express any protein even though it contains one exon of the VAMP1 gene. The replacement did not affect MDV replication in vitro, despite the virus having only one copy of ICP4. Furthermore, we have shown that the variant MDV genome containing the duck genome substitution is present in the parental Md11 population and has been maintained through several subsequent propagations of the virus on chicken cells. This finding provides direct evidence that host genome acquisition by MDV actually occurs during virus replication, and that one or more such MDV genomes with host sequences may exist within MDV viral stocks which tend to be polyclonal, due to the cell-associated nature of its infection process.

Single-nucleotide polymorphisms in two Marek's disease virus genes (Meq and gD): application to a retrospective molecular epidemiology study (1982–1999) in France

Marek's disease virus (MDV) is a herpesvirus that causes a lymphoproliferative disease in chickens. Vaccines against MDV are available, but the virus is gradually becoming more virulent. A molecular epidemiology study of MDV was carried out by assessing nucleotide variation in two different genes, Meq and gD, in 68 French field isolates circulating from 1982 to 1999, compared with reference strains. Viral DNA was amplified by nested PCR and sequenced directly. Comparison of the nucleotide sequences revealed a high nucleotide sequence identity (98 %). Single-nucleotide polymorphisms were identified, leading to the identification of three gene alleles for gD and six for Meq. Nine combinations of alleles were identified. A majority of French isolates (60·5 %) clustered in the C1 type, which has been present for over 17 years. Waves of non-C1-type isolates appeared when vaccine efficacy decreased. Furthermore, specific discriminating sequences were obtained for the CVI-988 vaccine strain.

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.

Transcriptional analysis of Marek's disease virus glycoprotein D, I, and E genes: gD expression is undetectable in cell culture

The various alphaherpesviruses, including Marek's disease virus (MDV), have both common and unique features of gene content and expression. The entire MDV U(s) region has been sequenced in our laboratory (P. Brunovskis and L. F. Velicar, Virology 206:324-338, 1995). Genes encoding the MDV glycoprotein D (gD), glycoprotein I (gI), and glycoprotein E (gE) homologs have been found in this region, although no gG homolog was found. In this work, transcription of the tandem MDV gD, gI, and gE genes was studied and found to have both unique characteristics and also features in common with other alphaherpesviruses. MDV gD could not be immunoprecipitated from MDV GA-infected duck embryo fibroblast cells by antisera reactive to its TrpE fusion proteins, while gI and gE could be. When the gD gene was subjected to in vitro-coupled transcription-translation, the precursor polypeptide was produced and could be immunoprecipitated by anti-gD. Northern blot, reverse transcriptase PCR, and RNase protection analyses have shown that (i) no mRNA initiating directly from the gD gene could be detected; (ii) a large but low-abundance 7.5-kb transcript spanning five genes, including the one encoding gD, was seen on longer exposure; and (iii) transcription of the gI and gE genes formed an abundant bicistronic 3.5-kb mRNA, as well as an abundant 2.0-kb gE-specific mRNA. Therefore, the MDV gD gene expression is down-regulated at the transcription level in MDV-infected cell culture, which may be related to the cell-associated nature of MDV in fibroblast cells. Compared to the highly gD-dependent herpes simplex virus and the other extreme of the varicella-zoster virus which lacks the gD gene, MDV is an intermediate type of alphaherpesvirus.

Detection of avian oncogenic Marek's disease herpesvirus DNA in human sera

The avian herpesvirus Marek's disease virus (MDV) has a worldwide distribution and is responsible for T-lymphoma in chickens. The question as to whether MDV poses a public health hazard to humans was first raised when the virus was isolated in 1967. However, no irrefutable results have been obtained in immunological and virological studies. We used a nested-PCR to detect MDV DNA in human serum samples. A total of 202 serum samples from individuals exposed and not exposed to poultry was tested by nested-PCR for a target sequence located in the MDV gD gene. The assay system was specific and sensitive, making it possible to detect a single copy of the target sequence. Forty-one (20%) of the 202 serum samples tested positive for MDV DNA. The prevalence of MDV DNA was not significantly different in the group exposed to poultry and the group not exposed to poultry. There was also no difference due to age or sex. Alignment of the 41 gD sequences amplified from human sera with eight gD sequences amplified from MDV-infected chicken sera showed a maximum nucleotide divergence of 1.65%. However, four 'hot-spot' mutation sites were identified, defining four groups. Interestingly, two groups contained only human MDV-gD sequences. The status of the MDV genome detected in human blood is discussed.

Reconstitution of Marek's disease virus serotype 1 (MDV-1) from DNA cloned as a bacterial artificial chromosome and characterization of a glycoprotein B-negative MDV-1 mutant

The complete genome of Marek's disease virus serotype 1 (MDV-1) strain 584Ap80C was cloned in Escherichia coli as a bacterial artificial chromosome (BAC). BAC vector sequences were introduced into the U(S)2 locus of the MDV-1 genome by homologous recombination. Viral DNA containing the BAC vector was used to transform Escherichia coli strain DH10B, and several colonies harboring the complete MDV-1 genome as an F plasmid (MDV-1 BACs) were identified. DNA from various MDV-1 BACs was transfected into chicken embryo fibroblasts, and from 3 days after transfection, infectious MDV-1 was obtained. Growth of MDV-1 recovered from BACs was indistinguishable from that of the parental virus, as assessed by plaque formation and determination of growth curves. In one of the MDV-1 BAC clones, sequences encoding glycoprotein B (gB) were deleted by one-step mutagenesis using a linear DNA fragment amplified by PCR. Mutant MDV-1 recovered after transfection of BAC DNA that harbored a 2.0-kbp deletion of the 2.6-kbp gB gene were able to grow and induce MDV-1-specific plaques only on cells providing MDV-1 gB in trans. The gB-negative virus reported here represents the first MDV-1 mutant with a deletion of an essential gene and demonstrates the power and usefulness of BACs to analyze genes and gene products in slowly growing and strictly cell-associated herpesviruses.

Recombinant Marek's disease virus (MDV)-derived lymphoblastoid cell lines: regulation of a marker gene within the context of the MDV genome

Marek's disease is a herpesvirus (Marek's disease virus [MDV])-induced pathology of chickens characterized by paralysis and the rapid appearance of T-cell lymphomas. Lymphoblastoid cell lines (LBCLs) derived from MDV-induced tumors have served as models of MDV latency and transformation. We have recently reported the construction of mutant MDVs having a deletion (M. S. Parcells et al., J. Virol. 69:7888-7898, 1995) and an insertion (A. S. Anderson et al., J. Virol. 72:2548-2553, 1998) within the unique short region of the virus genome. These mutant MDVs retained oncogenicity, and LBCLs have been established from the mutant-induced tumors. We report the characterization of these cell lines with respect to (i) virus structure within and reactivated from the cell lines, (ii) surface antigen expression, (iii) kinetics of MDV and marker gene induction, (iv) localization and colocalization of induced MDV antigens and beta-galactosidase (beta-Gal), and (v) methylation status of the region of lacZ insertion in recombinant- and non-recombinant-derived cell lines. Our results indicate that (i) recombinant-derived cell lines contain no parental virus, (ii) the established cell lines are predominantly CD4(+) CD8(-), (iii) the percentage of Lac-expressing cells is low (1 to 3%) but increases dramatically upon 5'-iododeoxyuridine (IUdR) treatment, (iv) lacZ expression is induced with the same kinetics as several MDV lytic-phase genes (pp38, US1, gB, gI, and US10), and (v) the regulation of lacZ expression is not mediated by methylation. Furthermore, the MDV-encoded oncoprotein, Meq, could be detected in cells expressing beta-Gal and various lytic antigens but did not appear to be induced by IUdR treatment. Our results indicate that regulation of the lacZ marker gene can serve as sensitive measure of virus lytic-phase induction and the reactivation from latency.

The genetic organization and transcriptional analysis of the short unique region in the genome of nononcogenic Marek's disease virus serotype 2

Studies on the Marek's disease virus (MDV) serotype 2 (MDV2) genome may be important for understanding the naturally nononcogenic nature of the virus. To determine the complete DNA sequence of MDV2 unique short (Us) region, genomic BamHI fragments F, M1 and R were sequenced. The MDV2 Us region is 12109 bp long and contains 12 potential open reading frames (ORFs) likely to encode for proteins. Seven of them exhibit homologies to herpes simplex virus type 1 (HSV-1) US1 (ICP22), US2, US3 (protein kinase), US6 (gD), US7 (gI), US8 (gE) and US10 genes. These ORFs are conserved in a similar arrangement with those of HSV-1, except for US10 which is transposed in the Us regions of all three MDV serotypes. The predicted amino acid sequence of MDV2 ORF6 is homologous to SORF3 of the other serotypes of MDV serotype 1 (MDV1) and herpesvirus of turkeys (HVT) and to infectious laryngotracheitis virus SR1. In addition, four ORFs, which have been identified around the Us and inverted repeat junction regions, have no apparent relation to any other known herpesvirus genes. The identified ORFs in the MDV2 Us region were more colinear with their previously reported locations of MDV1 than with those of HVT and other alphaherpesviruses. Ten of the 12 ORFs in the MDV2 Us region were expressed and transcribed with 3'-coterminal transcripts and/or a unique transcript in the virus-infected cells. Compared to other MDV serotypes, the MDV2 Us-encoded proteins showed 46-70% and 33-59% identities with equivalent of MDV1 and HVT at the amino acid level, respectively. Our present data will be useful to understand the different pathogenicity among serotypes of MDV and to allow precise manipulation of the genes for a possible use in genetically engineered vaccines.