Patent Publication Number: US-5252717-A

Title: Marek&#39;s disease herpesvirus DNA segments encoding glycoproteins, gD, gI and gE

Description:
This is a divisional of copending application Ser. No. 07/572,711 filed on Aug. 24, 1990 now U.S. Pat. No. 5,138,033. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to segments of the Marek&#39;s Disease Herpesvirus genome, from its unique short (U S ) region encoding glycoproteins gD, gI and part of gE, and to novel glycoproteins produced therefrom. In particular, the present invention relates to DNA segments encoding these glycoprotein antigens and the 5&#39; regulatory region of their genes, segments which are useful for probing for Marek&#39;s disease herpesvirus, as a possible source for MDV promoters, for gene expression to produce the glycoproteins that in turn can be used for producing antibodies which recognize the three glycoprotein antigens, and in the case of the latter two genes, for potential insertion sites for foreign genes. 
     (2) Prior Art 
     Marek&#39;s disease virus (MDV) is an oncogenic herpesvirus of chickens, which is known to cause T cell lymphomas and peripheral nerve demyelination. The resulting disease, Marek&#39;s disease (MD), was the first naturally occurring lymphomatous disorder to be effectively controlled via vaccination with the antigenically related, yet apathogenic herpesvirus of turkeys (HVT). 
     Because of similar biological properties, especially its lymphotropism, MDV has been classified as a member of the gammaherpesvirus subfamily (Roizman, B., et al., Intervirology 16:201-217 (1981)). Of the three herpesvirus subfamilies, gammaherpesviruses exhibit particularly marked differences with regard to genome composition and organization. For example, the B-lymphotropic Epstein-Barr virus (EBV) of humans has a 172.3 kbp genome with 60% G+C content, is bounded by terminal 0.5 kbp direct repeats and contains a characteristic set of internal 3.07 kbp tandem repeats (Baer, R., et al., Nature (London) 310:207-211 (1984)). Herpesvirus saimiri (HVS), a T-lymphotropic herpesvirus of new-world monkeys and lower vertebrates, has an A+T rich coding sequence (112 kbp; 36% G+C; i.e. L-DNA) without any large-scale internal redundancy, but contains instead greater than 30 reiterations of a 1.44 kbp sequence of 71% G+C at the termini of the genome (H-DNA) (Banker, A. T., et al., J. Virol. 55:133-139 (1985)). Despite the structural differences between EBV and HVS, the genomes of these two viruses encode serologically related proteins and share a common organization of coding sequences which differs from that of the neurotropic alphaherpesviruses, exemplified by herpes simplex virus (HSV) and varicella-zoster virus (VZV) (Camerion, K. R., et al., J. Virol. 61:2063-2070 (1987); Davison, A. J., et al., J. Gen. Virol. 68:1067-1079 (1987); Davison, A. J., et al., J. Gen. Virol. 67:597-611 (1986; Davison, A. J., et al., J. Gen. Virol. 76:1759-1816 (1986); Davison, A. J., et al., J. Gen. Virol. 64:1927-1942 (1983); Gompels, U. A., J. of Virol. 62:757-767 (1988); and Nichols, J., et al., J. of Virol. 62:3250-3257 (1988)). 
     In contrast to other gammaherpesviruses, MDV has a genome structure closely resembling that of the alphaherpesviruses (Cebrian, J., et al., Proc. Natl. Acad. Sci. USA 79:555-558 (1982); and Fukuchi, K., et al., J. Virol. 51:102-109 (1984)). Members of the latter subfamily have similar genome structures consisting of covalently joined long (L) and short (S) segments. Each segment comprises a unique (U) segment (U L , U S ) flanked by a pair (terminal and internals) of inverted repeat regions (TR L , IR L  ; TR S , IR S  ; respectively). Alphaherpesviruses include human HSV and VZV, porcine pseudorabies virus (PRV), bovine herpesvirus (BHV) and equine herpesvirus (ERV). Because MDV contains extensive repeat sequences flanking its U L  region, its genome structure most resembles that of HSV (Cebrian, J., et al., Proc. Natl. Acad. Sci. USA 79:555-558 (1982); and Fukuchi, K., et al., J. Virol. 51:102-109 (1984)). 
     Recent studies (Buckmaster, A. E., et al., J. Gen. Virol. 69:2033-2042 (1988)) have shown that the two gammaherpesviruses, MDV and HVT, appear to bear greater similarity to the alphaherpesviruses, VZV and HSV, than to the gammaherpesvirus, EBV. This was based on a comparison of numerous randomly isolated MDV and HVT clones at the predicted amino acid level; not only did individual sequences exhibit greater relatedness to alphaherpesvirus genes than to gammaherpesvirus genes, but the two viral genomes were found to be generally collinear with VZV, at least with respect to the unique long (U L ) region. Such collinearity of U L  genes extends to other alphaherpesviruses such as HSV-1, HSV-2, EHV-1 and PRV as evidenced by both sequence analysis (McGeoch, D. J., et al., J. Gen. Virol. 69:1531-1574 (1988)) and DNA hybridization experiments (Davison, A. J., et al., J. Gen. Virol. 64:1927-1942 (1983)). Many of these U L  genes are shared by other herpesviruses, including the beta- and gammaherpesviruses (Davison, A. J., et al., J. Gen. Virol. 68:1067-1079 (1987)). The organization and comparison of such genes has suggested the past occurrence of large-scale rearrangements to account for the divergence of herpesviruses from a common ancestor. Unfortunately, such a hypothesis fails to account for the presence of alphaherpesvirus S component (unique short, U S , and associated inverted/terminal repeat short, IR S , TR S ) genes which appear unique to members of this subfamily (Davison, A. J., et al., J. Gen. Virol. 68:1067-1079 (1987); Davison, A. J., et al., J. Gen. Virol. 67:597-611 (1986; and McGeoch, D. J. , et al., J. Mol. Biol. 181:1-13 (1985)). 
     In addition to its uniqueness compared with beta- and gammaherpesviruses, the alphaherpesvirus U S  region is particularly interesting because of marked differences in its content and genetic organization within the latter subfamily (eg HSV-1 US=13.0 kbp, 12 genes, McGeoch, D. J., et al., J. Mol. Biol. 181:1-13. (1985); VZV US=5.2 kbp, 4 genes, Davison, A. J., et al., J. Gen. Virol. 76:1759-1816 (1986)). In the case of HSV-1, 11 of the 12 US genes have been found to be dispensable for replication in cell culture (Longnecker, R., et al., Proc. Natl. Acad. Sci. USA 84:4303-4307 (1987)). This has suggested the potential involvement of these genes in pathogenesis and/or latency (Longnecker, R., et al., Proc. Natl. Acad. Sci. USA 84:4303-4307 (1987); Meignier, B., et al., Virology 162:251-254 (1988); and Weber, P. C., et al., Science 236:576-579 (1987)). In the report by Buckmaster et al. (Buckmaster, A. E., et al., J. Gen. Virol. 69:2033-2042 (1988)), except for the identification of partial MDV sequences homologous to HSV immediate early protein α22 (US2) and the serine-threonine protein kinase (US3), the content, localization and organization of MDV S component homologs was not determined. Moreover, despite the presence of at least four HSV US glycoprotein genes (two in VZV), no such homologs were identified. 
     In application Ser. No. 07/229,011 filed Aug. 5, 1988, including Leland F. Velicer, one of the present inventors, the Marek&#39;s Disease herpesvirus DNA encoding the glycoprotein B antigen complex (gp100, gp60, gp49) was identified but not sequenced. Antigen B is an important glycoprotein complex because it can elicit at least partial protective immunity, and the gene can be used for probes, as a possible source for promoters in its 5&#39; regulatory region, and for gene expression to produce the glycoproteins, which in turn can be used to produce antibodies that recognize the glycoprotein antigens. However, there was no discussion of the glycoproteins of the present invention. 
     In application Ser. No. 07/526,790, filed May 17, 1987 by Leland F. Velicer, the Marek&#39;s Disease herpesvirus DNA encoding the glycoprotein A antigen is described but not sequenced. This DNA is useful as probes, as a possible source for promoters in its 5&#39; regulatory region, and for producing antibodies by the sequence of events described above. This DNA is also important because antigen A is now known to be a homolog of HSV gC, a gene non-essential for replication in cell culture. Since that property most likely also applies to the MDV homolog, it may be useful as a site for insertion of foreign genes. However, there was no discussion of the glycoproteins of the present invention. 
     Little is know about the other glycoproteins produced by Marek&#39;s disease herpesvirus. The present invention is directed to the glycoproteins gD, gI and gE. 
     OBJECTS 
     It is an object of the present invention to provide sequenced DNA encoding glycoproteins gD, gI and part of gE, both together and individually. It is further an object of the present invention to provide DNA segments encoding these glycoprotein antigens and the up to 400 nucleotides 5&#39; regulatory regions of their genes; which are useful as DNA probes, as a possible source for MDV promoters, for producing antibodies which recognize the antigens and, in the case of the latter two glycoproteins, as likely insertion sites for foreign genes. These and other objects will become increasingly apparent by reference to the following description and the drawings. 
    
    
     IN THE DRAWINGS 
     FIG. 1 shows map location, sequencing strategy and organization of MDV open reading frames (ORFs): 
     Part A includes MDV genomic structure and restriction maps outlining area sequenced. 
     Part B includes sequencing strategy. R and L represent rightward- and leftward-directed sequences from M13 clones. The restriction enzyme sites are identified as: B=BamHI, E=EcoRI, N=NcoI, and P=PstI. Sequences derived from random libraries (Sau3A, TaqI, RsaI), specific cloned restriction fragments, Bal31-digested libraries, and using synthetically-derived oligonucleotides are denoted by a, b, c, and d, respectively. 
     Part C includes organization of nine MDV U S  ORFs. Numbers refer to homologs based on relation to HSV-1 U S  ORF nomenclature (McGeoch, D. J., et al., J. Mol. Biol. 181:1-13. (1985)). SORF1 and SORF2 are MDV-specific S component ORFs given arbitrary names. Upper case and lower case solid bars refer to rightward and leftward-directed ORFs, respectively. 
     FIGS. 2a to 2h show nucleotide and predicted amino acid sequences. The nucleotide sequence is given as the rightward 5&#39; to 3&#39; strand only (numbered 1 to 8799). Rightward- and leftward-directed predicted amino acid sequences are shown above and below the corresponding nucleotide sequences in single-letter code, respectively. The name of each ORF is given to the left of the first line of the amino acid sequence. Amino acid sequences are numbered from the first M (ATG in the DNA) at the N terminus to the last amino acid at the C-terminus, which precedes the termination codon (identified by a *). Potential TATA consensus sites located within 400 nucleotides of the ATG are underlined and defined as sites containing at least six of seven matches to the TATA(AT)A(AT) consensus sequences defined by Corden et al. (Corden, B., et al., Science 209:1406-1414 (1980)). Underlines longer than seven nucleotides long refer to areas containing overlapping TATA consensus sites. 
     FIG. 3A shows alignment of S component homologs showing selected regions displaying maximum amino acid conservation. Gaps have been introduced to maximize alignment of identical amino acids as described in Methods. The consensus sequence (cons) indicates residues that are shared by at least all but one of the viruses and are indicated by capital letters. In alignments between more than two sequences, asterisks (*) indicate residues conserved by all of the viruses. Amino acid numbers (with respect to 5&#39;-ATG) of corresponding regions aligned are listed before and after each sequence. The HSV-1 US8 homolog was not aligned in order to emphasize the relatedness between the other US8 homologs. 
     FIG. 3B shows the dot matrix analyses depicting overall homologies between selected MDV-alphaherpesvirus S segment homolog comparisons. Points were generated where at least 15 amino acids over a sliding window length of 30 were found identical or similar. The resulting diagonals illustrate regions showing greatest conservation. Amino acid numbers (with respect to 5&#39;-ATG) of corresponding sequences are denoted above and to the right of each plot. 
     FIG. 4 shows a comparison of overall genome organization of available S component ORFs (Cullinane, A. A., et al., J. Gen. Virol. 69:1575-1590 (1988); Davison, A. J., et al., J. Gen. Virol. 76:1759-1816 (1986); McGeoch, D. J., et al., J. Mol. Biol. 181:1-13. (1985) Petrovskis, E. A., et al., Virology 159:193-195 (1987); Petrovskis, E. A., et al., J. Virol. 60:185-193 (1986) ; and Petrovskis, E. A., et al., J. Virol. 59:216-223 (1986)). Numbers above each ORF refer to homologs based on relation to HSV-1 U S  ORF nomenclature (McGeoch, D. J., et al., J. Mol. Biol. 181:1-13. (1985)). Alternative polypeptide designations common to each system are listed below those ORFs where applicable. Upper and lower case solid bars refer to rightward and leftward-directed ORFs, respectively. Arrows refer to identified IR S  -U S  and/or U S  -TR S  junction sites. 
     FIG. 5 shows the sequence of steps necessary to produce a complete segment of Marek&#39;s disease herpesvirus DNA encoding glycoprotein gI and part of gE. 
    
    
     GENERAL DESCRIPTION 
     The present invention relates to a segment of DNA of Marek&#39;s disease herpesvirus genome encoding multiple glycoproteins, and containing potential promoter sequences up to 400 nucleotides 5&#39; of each gene, between a 1 and 8799 nucleotide sequence as shown in FIG. 2 (and identified as SEQ ID No: 1 ). 
     Further the present invention relates to an EcoR1 I segment of Marek&#39;s disease herpesvirus genome encoding the glycoprotein D precursor, and subsegments of the DNA. 
     Further still, the present invention relates to a segment of DNA encoding glycoprotein gD precursor between a 5964 and 7175 nucleotide sequence of Marek&#39;s disease herpesvirus DNA, and the gene&#39;s 5&#39; regulatory region of up to 400 nucleotides in length, as shown in FIG. 2 and subsegments of the segment of DNA which recognize the DNA. 
     The present invention also relates to a segment of DNA encoding glycoprotein gI precursor between a 7282 and 8346 nucleotide sequence of Marek&#39;s disease herpesvirus DNA, and the gene&#39;s 5&#39; regulatory region of up to 400 nucleotides in length, as shown in FIG. 2 and subsegments of the segments that recognize the DNA. 
     The present invention also relates to a segment of DNA encoding a part glycoprotein gE precursor between a 8488 and 8799 nucleotide DNA sequence of Marek&#39;s disease herpesvirus, and the gene&#39;s 5&#39; regulatory region of up to 400 nucleotides in length, as shown in FIG. 1 and subfragments of the DNA that recognize the DNA. 
     Further the present invention relates to the novel glycoprotein precursors which are produced by expression of the genes in the segments of DNA. 
     Further the present invention relates to the potential MDV gene promoters, which are located in the 400 nucleotides 5&#39; of each coding sequence. 
     SPECIFIC DESCRIPTION 
     The present invention shows a sequence analysis of an 8.8 kbp DNA stretch encompassing a majority of the MDV U S  region. Altogether seven MDV U S  homologs, including three glycoprotein genes and two additional MDV-specific open reading frames, were identified. 
     EXAMPLE 1 
     Materials and Methods 
     Recombinant plasmids, M13 subcloning and DNA sequencing MDV EcoR1-O and EcoR1-I of the pathogenic GA strain were previously cloned into pBR328 by a graduate student at Michigan State University, East Lansing, Mich. (Gibbs, C. P., et al., Proc. Natl. Acad. Sci. USA 81:3365-3369 (1984) and Silva, R. F., et al., J. Virol. 54:690-696 (1985)) and made available by R. F. Silva, USDA Regional Poultry Research Lab, East Lansing, Mich., where these clones are maintained. GA strain BamHI-A and BamHI-P1 were previously cloned into pACYC184 and pBR322, respectively (Fukuchi, K., et al., J. Virol. 51:102-109 (1984)) and kindly provided by M Nonoyama, Showa University Research Institute, St. Petersburg, Fla. Small- and large-scale plasmid preparations were made using the alkaline lysis procedure (Maniatis, T., et al., Molecular cloning: a laboratory manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1982)). 
     M13mp18 and M13mp19 phage subclones to be used as templates for sequencing were generated using specific restriction subfragments determined by restriction mapping or the use of Sau3A, Taq I or RsaI-digested viral DNA pools ligated into the unique BamHI, AccI or SmaI sites of M13 RF DNA, respectively. In some cases overlapping M13 deletion clones were obtained by processive Ba131 digestions from AccI, NaeI or NsiI restriction sites in EcoR1-O by the method of Poncz et al (Poncz, M., et al., Proc. Natl. Acad. Sci. USA 79:4298-4302 (1982)). Standard methods (Maniatis, T., et al., Molecular cloning: a laboratory manual. Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y. (1982)) were used for restriction digestions, gel electrophoresis, purification of DNA fragments from agarose gels, ligations and fill-in of 5&#39; overhangs with Klenow fragment. 
     Ligated M13 products were transformed into CaCl 2  -competent JM107 host cells and added to melted B top agar containing 10 μl of 100 mM IPTG, 50 μl of 2% X-gal and 200 μl of a fresh overnight JM101 culture. These contents were then plated onto B agar plates and incubated at 37° C. overnight. Recombinant (clear) plaques were then used to infect 5 ml of YT media diluted 1:50 with an overnight JM101 culture and rotated at 37° C. for 6 hours. The resulting cells were pelleted by centrifugation for 5 minutes at room temperature and the supernatants were removed and stored at 40° C. to retain viral stocks of each recombinant clone. 
     Using the recovered supernatants, Single-stranded M13 phage DNA to be used as templates for DNA sequencing by the dideoxy-chain termination method was isolated according to instructions in the M13 Cloning/Dideoxy Sequencing Instruction Manual provided by Bethesda Research Laboratories. Recombinant M13mp phages were further screened by electrophoresing purified single-stranded viral DNA on 1% agarose mini-gels and selecting those templates showing reduced mobility in comparison to single-stranded M13mp 18 control DNA. 
     DNA sequencing with single-stranded M13 templates was performed by the dideoxy-chain termination method (Sanger, F. S., et al., Proc. Natl. Acad. Sci. USA 74:5463-5467 (1977)) employing the modified T7 DNA polymerase, Sequenase™ (United States Biochemical Corp., Cleveland, Ohio). A summary of the sequencing strategy is included in FIG. 1B. For DNA sequencing reactions, the specific step by step instructions provided with the Sequenase™ sequencing kit were employed. Briefly, single-stranded M13 templates were first annealed with the universal M13 synthetic oligonucleotide primer by incubation at 650° C. for 2 minutes followed by slow cooling until the incubation temperature was below 30° C. Following the addition of proper mixtures of deoxy- and dideoxynucleotide triphosphates (dNTPs and ddNTPs, respectively), radioactively labeled deoxyadenosine 5&#39;- (α-thio) triphosphate ( 35  S-dATP, 1000-1500 Ci/mmol; NEN-DuPont) and the Sequenase™ enzyme, synthesis of radioactively labeled complementary strands was initiated from the annealed primer. Four separate synthesis reactions were each terminated by the incorporation of the specific ddNTP (ddATP, ddGTP, ddTTP or ddCTP) used in each tube. Reaction products were electrophorsed through 7% polyacrylamide/50% urea/Tris-Borate-EDTA gels and the labeled chains were visualized by autoradiography. Both strands were sequenced at least once. This was facilitated by the use of 16 synthetic 17-mer oligonucleotides generated based on previously determined sequences and substituted for the universal primer under similar reaction conditions above (0.5 pmoles/reaction) according to the general approach described by Strauss (Strauss, E. C., et al., Anal. Biochem. 154:353-360 (1986)). 
     Analysis of sequence data 
     Sequences were assembled and analyzed on an IBM personal System 2/Model 50 microcomputer utilizing the IBI/Pustell (Pustell, J., et al., Nucl. Acids. Res. 14:479-488 (1986)) and Genepro (Version 4.10; Riverside scientific Enterprises, Seattle, Wash.) sequence analysis software packages or programs obtained from the University of Wisconsin Genetics Computer Group (GCG; Devereaux, J., et al., Nucl. Acids. Res. 12:387-395. (1984)) and run on a VAX 8650 minicomputer. Database searches of the National Biochemical Research Foundation-Protein (NBRF-Protein, Release 21.0, 6/89) were made with the GCG program FASTA (Pearson, W. R., et al., Proc. Natl. Acad. Sci. USA 85:2444-2448 (1988)) which uses: (1) a modification of the algorithm of Wilbur and Lipman (Wilbur, W. J., et al., Proc. Natl. Acad. Sci. USA 80:726-730 (1983)) to locate regions of similarity; (2) a PAM250-based scoring system (Dayhoff, M. O., et al., p. 345-352. In M. O. Dayhoff (ed.), Atlas of protein sequence and structure, vol. 5, Suppl. 3. National Biomedical Research Foundation, Washington, D.C. (1978)) and (3) the alignment procedure of Smith and Waterman (Smith, T. F., et al, Adv. Appl. Mathematics 2:482-489 (1981) to join together, when possible, the highest-scoring, non-overlapping regions in order to derive an alignment and its resulting, optimized score. Dot matrix homology plots were generated by using the GCG program DOTPLOT with the output file from GCG&#39;s COMPARE. The latter creates a file of the points of similarity between two predicted amino acid sequences for which a window length of 30 and a stringency of 15 (in which conservative amino acid replacements are scored positive) were chosen. Using the GCG program GAP, specific amino acid sequences were aligned using the algorithm of Needleman and Wunsch (Needleman, S. B., et al., J. Mol. Biol. 48:443-453 (1970)); following the insertion of gaps (to maximize the number of matches) the percentage of identical and similar amino acid residues were determined. To create multiple alignments using GAP, output files of gapped MDV sequences were created following successive GAP comparisons between the MDV sequence and its homologous sequences (in descending order of homology). These output files were used as input sequences for subsequent runs of GAP until the alignment of these gapped sequences could no longer be expanded by the addition of new gaps. Following alignment, the gapped output files were displayed and a consensus sequence calculated using the GCG program PRETTY. To achieve optimal results, in some cases manual editing was employed (using GCG&#39;s LINEUP). 
     Results 
     The 8,799 nucleotide DNA sequence presented (FIG. 2) appears to encompass a majority of the MDV (GA) genome&#39;s unique short (US) region. A summary of the sequencing strategy is included in Materials and Methods and is depicted in FIG. 1B. This sequence spans the US fragments, EcoR1-O, EcoR1-I and extends to the end of BamHI-P 1 , (FIGS. 1A and B). Fukuchi et al. (Fukuchi, K., et al., J. Virol. 51:102-109 (1984)) have previously mapped the IR S  -U S  junction to a 1.4 kb Bgl I fragment located in the second of five EcoR1 subfragments of BamHI-A (FIG. 1B). Thus, the sequence presented here should lack between 2.6 and 4.0 kb of the 5&#39;-proximal U S  region. Because of the lack of available MDV clones mapping downstream of BamHI-P 1 , the U S  -TR S  junction has not yet been precisely defined (Davison, A. J., et al., J. Gen. Virol. 76:1759-1816 (1986)). For VZV, EHV-4 and HSV-1, this border is located about 100 bp upstream or 1.1 and 2.7 kb downstream, respectively, of the termination codon of their respective US8 homologs (Cullinane, A. A., et al., J. Gen. Virol. 69:1575-1590 (1988); Davison, A. J., et al., J. Gen. Virol. 76:1759-1816 (1986); and McGeoch, D. J., et al., J. Gen. Virol. 69:1531-1574 (1988)). 
     The overall G+C content of the region sequenced was found to be 41%, somewhat below the genomic MDV G+C values of 46% (Lee, L. F., et al., J. Virol. 7:289 (1971)). Observed frequencies of CpG dinucleotides in the whole sequence, or in the coding regions only, did not differ significantly from those expected from their mononucleotide compositions (data not shown). This result agrees with those obtained from alphaherpesviruses, while contrasting with those obtained from gammaherpesviruses, such as the A+T rich HVS and the G+C rich EBV, which are both deficient in CpG dinucleotides (Honess, R. W., et al., J. Gen. Virol. 70:837-855 (1989)). 
     The region sequenced contains 1 partial and 8 complete ORFs likely to code for proteins (FIG. 1C, basis for names is given below). This prediction was based on: (1) homology and positional organization comparisons to other alphaherpesvirus genes and (2) presence of potential TATA and polyadenylation consensus sequences (Birnstiel, M. L., et al., Cell 41:349-359 (1985); and Corden, B., et al., Science 209:1406-1414 (1980) ), and (3) possession of favorable contexts for translational initiation (Kozak, M., J. Cell Biol. 108:229-241 (1989)). This identification was further guided by the observation that alphaherpesviruses such as HSV and VZV tend to contain relatively tightly packed, unspliced and generally nonoverlapping coding regions (Davison, A. J., et al., J. Gen. Virol. 76:1759-1816 (1986); Davison, A. J., et al., J. Gen. Virol. 76:1759-1816 (1986); McGeoch, D. J., et al., J. Gen. Virol. 69:1531-1574 (1988); McGeoch, D. J., et al., J. Mol. Biol. 181:1-13. (1985); and McGeoch, D. J., et al., J. Gen. Virol. 68:19-38 (1987)). Such genes, especially those of the U S  regions, often share polyadenylation signals, thereby resulting in 3&#39;-coterminal mRNA families (Rixon, F. J., et al., Nucl. Acids Res. 13:953-973 (1985)). Methods for detecting protein coding regions based on the use of MDV-derived codon frequency tables (using these and previously published MDV sequences, Binns, M. M., et al., Virus Res. 12:371-382 (1989); Ross, L.J.N., et al., J. Gen. Virol. 70:1789-1804 (1989); and Scott, S. D., et al., J. Gen. Virol. 70:3055-3065 (1989)) or analysis of compositional bias (using the GCG programs CODONPREFERENCE and TESTCODE) were largely inconclusive, suggesting that MDV possesses relatively low codon and compositional biases compared to those predicted based on its mononucleotide composition. However, using the GCG program FRAMES, together with the MDV-derived codon frequency table above, the 9 identified ORFs clearly show a significantly low pattern of rare codon usage, which sharply contrasts with that observed in all other potentially translatable regions (data not shown). 
     The predicted amino acid sequences of the predicted ORFs (beginning from the first ATG codon) are shown relative to the nucleotide sequence in FIG. 2. Potential TATA sites within 400 nucleotides of the initiation codon are underlined. Proposed ORF and potential polyadenylation signal locations, identification of the -3, +4 ATG context nucleotides (Kozak, M., J. Cell Biol. 108:229-241 (1989)), as well as the lengths, relative molecular masses and predicted isoelectric points of the predicted translational products are shown in Table 1 (set forth at the end of the description). In the absence of previous information concerning these MDV ORFs, and to simplify identification, they have been named (FIG. 1C, Table 1) based on homologous relationships to HSV-1 encoded US ORFs (McGeoch, D. J., et al., J. Mol. Biol. 181:1-13. (1985)). When appropriate, the letters MDV will preface the homolog&#39;s name to indicate the ORF&#39;s origin. The two MDV-specific ORFs have been arbitrarily named SORF1 and SORF2, based on their location in the S component. 
     According to the scanning model for translation, the 40S ribosomal subunit binds initially at the 5&#39;-end of mRNA and then migrates, stopping at the first AUG (ATG) codon in a favorable context for initiating translation (Kozak, M., J. Cell Biol. 108:229-241 (1989)). However, in the absence of S1 nuclease and/or primer extension analysis, definitive start sites for translation cannot be accurately predicted. Nevertheless, likely start sites are listed in Table 1; these refer to the location of the first in-frame ATG codon found in the major open reading frame. According to Kozak (Kozak, M., J. Cell Biol. 108:229-241 (1989)), as long as there is a purine in position -3, deviations from the rest of the consensus only marginally impair initiation. In the absence of such a purine, however, a guanine at position +4 is essential for efficient translation. Table 1 shows that all of the ORFs, except for SORF2, contain the important purine residue in the -3 position. Nevertheless, in the case of SORF2, a compensating guanine in position +4 is indeed present. 
     In the case of MDV US1, two transcriptional cap sites have been tentatively identified by 5&#39; S1 nuclease protection analysis (data not shown). These Bites appear to be located 18 and 25 nucleotides downstream of a TATATAA sequence at position 200 and 207, respectively (FIG. 2). Based on 3&#39; S1 data, this transcript utilizes a polyadenylation signal located just downstream of the US10 coding region (Table 1, data not shown). Comparative Northern blot analyses of the US region indicate that the 14DV US1 transcript appears to be the most prominent transcript expressed at late times (72 h) post-infection when extensive cytopathic effects are observed (data not shown). Phosphonoacetic acid inhibition studies have indicated that MDV US1, in contrast to its immediate-early HSV1 US1 counterpart, is regulated as a late class gene (data not shown). 
     Using the computer program FASTA (Pearson, W. R., et al., Proc. Natl. Acad. Sci. USA 85:2444-2448 (1988)) with a K-tuple value of 1, each of the 9 predicted amino acid sequences was screened against the NBRF-Protein database (Release 21.0, 6/89), and recently published EHV-4 S segment gene sequences (11). Optimized FASTA scores of greater than 100 were considered to indicate a significant degree of amino acid similarity. The results of this analysis are in Table 2 (set forth at the end of the description). While SORF1 and SORF2 do not appear to share any significant homology to any of the sequences in the database (data not shown), apart from MDV US3, the other six ORFs (MDV US1, 10, 2, 6, 7, and 8; Tables 1, 2) were found to be homologous to alphaherpesvirus S segment genes exclusively (Table 2). Because the US3 ORF represents a member of the serine-threonine protein kinase superfamily (Hanks, S. K., et al., Science 241:42- (1988)), a relatively large number of scores above 150 were obtained. Nevertheless, these scores were 3-4 fold lower than those obtained in comparisons with US3 homologs of HSV and VZV. To compare with previously established alphaherpesvirus s segment homologies, all possible FASTA comparisons between the seven groups of alphaherpesvirus-related sequences are included. The program GAP was used in similar pairwise comparisons to generate optimal alignments in order to determine the total percentage of identical and similar amino acids shared by the two sequences. As shown in Table 2, homology comparisons between MDV S segment ORFs and their alphaherpesvirus counterparts were comparable to those previously observed between the other alphaherpesvirus S segment homologs themselves. In some cases MDV ORFs were found to be more related to alphaherpesvirus homologs than those same homologs were to their other alphaherpesvirus counterparts (compare MDV/EHV-4 vs. HSV-1/EHV-4 US1 and MDV/EHV-4 vs. HSV-1/EHV-4 US10 homologies). Moreover, despite the fact that VZV lacks US2 and US6 homologs, MDV, although formally considered a gammaherpesvirus, clearly does possess US2 and US6 homologs. With regard to US8 homologs, pairwise comparisons using GAP were limited to those regions optimally aligned with the partial MDV US8 sequence. Interestingly, the MDV, VZV and PRV US8 homologs exhibit a degree of relatedness over this region that is not shared with HSV-1. The results of limited multiple alignments for each of the seven homologs in which areas showing best conservation are depicted in FIG. 3A. 
     Dot matrix homology plots depicting overall homologies between selected MDV-alphaherpesvirus S segment homolog comparisons are included in FIG. 3B. (using a sliding window length of 30 amino acids, in which points are generated where at least 15 amino acids are found identical or similar.) The resulting diagonals illustrate the regions showing greatest conservation. Such regions include and in some cases extend upon those regions depicted in FIG. 3A. 
     More sensitive attempts to identify other related proteins not detected with FASTA were made using the GCG programs PROFILE and PROFILESEARCH. Use of these programs permit database comparisons which rely on information available from structural studies and, in this case, from information implicit in the alignments of related S component ORFs (including MDV sequences using GAP) (Gribskov, M., et al., Proc. Natl. Acad. Sci. USA 84:4355-4358 (1987)); nevertheless, such analyses failed to extend upon the groups of related proteins described here. 
     Herpesvirus glycoprotein homologs have generally been found to contain similar patterns of conserved cysteine residues. In comparing the gB homologs of seven different herpesviruses included in the alpha-, beta- and gammaherpesvirus subclasses, there is complete conservation of 10 cysteine residues (Ross, L. J. N., et al., J. Gen. Virol. 70:1789-1804 (1989)). HSV-1 US6(gD) contains 7 cysteine residues: six appear critical for correct folding, antigenic structure and extent of oligosaccharide processing (Wilcox, W. C., et al., J. Virol. 62:1941-1947 (1988)). Not only is this same general pattern of cysteines conserved in the gD homologs of HSV-2 (McGeoch, D. J., et al., J. Gen. Virol. 68:19-38 (1987)) and PRV (Petrovskis, E. A., et al., J. Virol. 59:216-223 (1986)), but they are conserved in the MDV gD homolog as well (full alignment not shown). FIG. 3A depicts portions of cysteine conservation patterns observed among the US6 (gD), US7 (gI), and US8 (gE) homologs (in which case 4, 3, and 4 conserved cysteine residues are shown, respectively). Interestingly, the basis for the relatively high degree of homology between gE homologs of MDV, VZV and PRV over the limited area described above is largely based on a unique, shared pattern of relatively rare cysteine residues which HSV-1 lacks. While the HSV-1, VZV and PRV gE homologs (and probably MDVs) are known to share six different cysteines located further downstream, it is quite possible that the unique pattern of four conserved cysteines could facilitate the formation of different secondary and tertiary structures which might impart important functional consequences. 
     Careful inspection of the N-terminal regions of the MDV gD, gI and gE homologs has revealed that they contain the three basic building blocks of signal peptide sequences: a basic, positively charged N-terminal region (n-region), a central hydrophobic region (h-region), and a more polar terminal region (c-region) that seems to define the cleavage site (von Heijne, G. J. Mol. Biol. 184:99-105 (1985)). Using a recently improved method for predicting signal sequence cleavage sites (von Heijne, G. Nucl. Acids Res. 14:4683-4690 (1986)), Table 3 (set forth at the end of the description) shows the likely position of these sites, the location of the hydrophobic transmembrane and charged cytoplasmic domains near the C-terminal end and the location of potential N-glycosylation sites. Like the other gI homologs, MDV&#39;s counterpart contains a relatively long cytoplasmic domain. However, in contrast to the other gD homologs, MDV gD&#39;s signal peptide contains a relatively long n-region (18 residues), that is unusually highly charged (+4; FIG. 2) considering an overall mean value of +1.7 among eukaryotes, which generally does not vary with length (von Heijne, G. J. Mol. Biol. 184:99-105 (1985)). Although a more distal methionine codon exists directly before the hydrophobic initiator codon (as in the PRV gD homolog, Petrovskis, E. A., et al., J. Virol. 59:216-223 (1986)) the scanning model for translation (Gribskov, M., et al., Proc. Natl. Acad. Sci. USA 84:4355-4358 (1987)) favors usage of the more 5&#39;-proximal initiation codon (at position 5964, FIG. 2). Further support is based on an overall translation context that appears at least as good, if not better than the one corresponding to the downstream ATG. Despite such a prediction, a possible MRNA cap site location between the two ATG sites, which would preclude such a prediction, cannot be ruled out at this point. 
     One final point concerning MDV gD requires mention. Using the 8,799 nucleotide DNA sequence as a probe for screening the GenBank (62.0, 12/89) and EMBL (19.0, 5/89) nucleic acid databases with the computer program FASTA (K-tuple=6), an optimized score of 1027, corresponding to 91.5% nucleotide identity in a 342 bp overlap between MDV gD coding sequences (6479-6814; aa#173--aa#284; FIG. 2) and a previously reported 467 bp MDV DNA segment (Wen, L.-T., et al., J. Virol. 62:3764-3771 (1988)). The latter sequence has been reported to contain a 60 bp segment protected against DNAase digestion by binding of a 28 kD MDV nuclear antigen (MDNA) expressed only in &#34;latently&#34; infected MDV-transformed lymphoblastoid cells. In view of similarities between MDV and VZV, these authors suggested is that MDNA may function in a manner analogous to that of EBNA-1 in immortalizing primate cells. In their report, Wen et al. (Wen, L.-T., et al., J. Virol. 62:3764-3771 (1988)) mapped the MDNA binding site to the same EcoRI subfragment of BamHI-A in which MDV gD is located (EcoRI-I, FIG. 1) Although our sequence covering this region is consistent with a complete, uninterrupted ORF containing all the characteristic features of a glycoprotein and showing significant homology to HSV gD, their sequence contains about 140 bases of 5&#39;-proximal sequence unrelated to any determined from our 5.3 kbp EcoR1-I fragment or its adjoining 3.5 kb sequences. The remaining 327 bp sequence (which contains the putative nuclear antigen binding site) while clearly resembling our gD coding sequence, upon computer translation fails to yield any ORF longer than 30 aa. 
     Discussion 
     Recent data have shown that despite MDV&#39;s classification as a gammaherpesvirus, based on lymphotropic properties shared with other members of this subfamily, its genome structure (Cebrian, J., et al., Proc. Natl. Acad. Sci. USA 79:555-558 (1982); and Fukuchi, K., et al., J. Virol. 51:102-109 (1984)) and genetic organization of primarily its U L  region (Buckmaster, A. E., et al., J. Gen. Virol. 69:2033-2042 (1988)) more closely resembles that of the neurotropic alphaherpesviruses. Moreover, in cases where polypeptide sequences were found conserved among the three herpesvirus subfamilies (eg. U L  genes), significantly higher homology scores were consistently observed against the respective alpha- rather than beta- or gammaherpesvirus counterparts (Davison, A. J., et al., J. Gen. Virol. 67:597-611 (1986); Buckmaster, A. E., et al., J. Gen. Virol. 69:2033-2042 (1988); Ross, L. J. N., et al., J. Gen. Virol. 70:1789-1804 (1989); and Scott, S. D., et al., J. Gen. Virol. 70:3055-3065 (1989)). Alphaherpesvirus S segment genes have previously been found to be unique to members of this taxonomic subfamily (Davison, A. J., et al., J. Gen. Virol. 68:1067-1079 (1987); and Davison, A. J., et al., J. Gen. Virol. 67:597-611 (1986). The identification of seven MDV homologs of alphaherpesvirus S segment genes in this study is consistent with the idea that MDV shares a closer evolutionary relationship with alphaherpesviruses than gammaherpesviruses. This is further supported by dinucleotide frequency analysis which fails to show a lack of CpG suppression as observed among all gammaherpesviruses thus far studied (Efstathiou, S., et al., J. Gen. Virol. 71:1365-1372 (1990); and Honess, R. W., et al., J. Gen. Virol. 70:837-855 (1989)). The above situation resembles a similar one observed with human herpesvirus-6 (HHV-6), in which case its T-lymphotropism suggested provisional classification as a gammaherpesvirus (Lopez, C., et al. , J. Infect. Dis. 157:1271-1273 (1988) ). However, subsequent genetic analysis has shown a greater relatedness between HHV-6 and the betaherpesvirus, human cytomegalovirus (HCMV; Lawrence, G. L., et al., J. Virol. 64:287-299 (1990)). 
     A comparison of the genetic organization of alphaherpesvirus S segment genes in presented in FIG. 4. The organization of these genes differ greatly in overall length, organization and degree of homology. Nevertheless, the overall gene layouts displayed are consistent with a model to account for the divergence of alphaherpesviruses from a common ancestor by a number of homologous recombination events which result in expansion or contraction of the inverted repeat regions and a concomitant loss or gain of U S  gene(s). In the case of VZV, six S segment homologs are lacking compared to HSV-1 (US2, US4, US5, US6, US11, US12). Some genes, such as the US1 homologs, show particular sequence and length divergences. Compared to HSV-1, the MDV, VZV and EHV-4 US1 homologs lack approximately 120 aa of sequence comparable to the 5&#39;-proximal portion of HSV-1 US1 (a22). Based on Northern blot analysis, S1 nuclease protection analysis and phosphonoacetic acid inhibition studies, in contrast to its relatively uncharacterized immediate-early HSV-1 counterpart, the MDV US1 gene appears to be regulated as an abundantly expressed late class gene (data not shown). In contrast to the other alphaherpesviruses, MDV contains two apparently MDV-specific ORFs. Moreover, the MDV U S  region appears to contain approximately 2.6 to 4.0 kb of additional 5&#39;-proximal sequences. Based on a comparison of FIG. 4 and consideration of the expansion-contraction recombination scheme, it appears likely that there are additional MDV-specific U S  genes. 
     Since MDV has long been regarded as a gammaherpesvirus, much of the previous work interpreting their properties has proceeded by analogy with the association between EBV and B cells (Nonoyama, M. p. 333-341. In B. Roizman (ed.), The herpesviruses, vol. 1. Plenum Press (1982); and Wilbur, W. J., et al., Proc. Natl. Acad. Sci. USA 80:726-730 (1983)). Because of a closer genetic relationship to the alphaherpesviruses, and keeping in mind the analysis of HHV-6 above we agree with Lawrence et al. (Lawrence, G. L., et al., J. Virol. 64:287-299 (1990)) that the lymphotropic properties of MDV and HVT are unlikely to be determined by molecules homologous to EBV and that a delineation of molecular differences between MDV and the neurotropic alphaherpesviruses would be more fruitful in explaining the observed biological differences than employing analogies based on properties of gammaherpesviruses such as EBV and HVS. 
     To account for such differences, the MDV U S  region may be particularly important. With few exceptions, each HSV-1 component gene possesses an equivalent in VZV (McGeoch, D. J., et al., J. Gen. Virol. 69:1531-1574 (1988)); a considerable number of these are related to beta- and gammaherpesvirus genes as well (29 of 67 EBV counterparts to VZV U L  genes; Davison, A. J., et al., J. Gen. Virol. 68:1067-1079 (1987)). In contrast, the S segments of HSV-1 and VZV differ significantly in size and appear to be among the least related parts of the two genomes (Davison, A. J., et al., J. Gen. Virol. 67:597-611 (1986; and Davison, A. J., et al., J. Gen. Virol. 64:1927-1942 (1983)). Recent studies have shown that 11 of 12 open reading frames contained in the HSV-1 S component are dispensable for growth in cell culture (Longnecker, R., et al., Proc. Natl. Acad. Sci. USA 84:4303-4307 (1987); and Weber, P. C., et al., Science 236:576-579 (1987)). The maintenance and evolution of such a dispensable gene cluster suggests the presence of functions relevant to the viruses survival in its specific ecological niche in the natural or laboratory animal host, rather than the presence of functions necessary for replication (Longnecker, R., et al., Proc. Natl. Acad. Sci. USA 84:4303-4307  (1987); and Weber, P. C., et al., Science 236:576-579 (1987) ) . Consistent with such a hypothesis are findings that HSV mutants carrying different S component gene-specific deletions were significantly less pathogenic and exhibited a reduced capacity for latency establishment in mice (Meignier, B., et al., Virology 162:251-254 (1988) ) . In regard to the latter, there is evidence suggesting that transcribed RNA from the HSV U S  region may be involved in the establishment and maintenance of an in vitro latency system employing human fetus lung fibroblast cells (Scheck, A. C., et al., Intervirology 30:121-136 (1989)). Taken together, the above evidence suggest(s) potentially important role(s) for MDV&#39;s U S  genes in tissue tropism, latency, and/or induction of cell transformation. 
     A consideration of the three gD, gI and gE homologs identified in this invention raises an interesting question. Fully enveloped infectious MDV virions are only known to be produced in feather follicle epithelial cells (Payne, L. N. p. 347-431. In B. Roizman (ed.), The herpesviruses, vol. 1. Plenum Press (1982)). Because of this, MDV studies have had to rely on limited fibroblast cell cultures which only promote the spread of cell-associated infections in vitro. Over the last 20 years, studies aimed at identifying immunogenic surface antigens have relied on this in vitro culture system and altogether only two glycoprotein antigens (A antigen/gC homolog; B antigen) have been routinely identified and characterized (Binns, M. M., et al., Virus Res. 12:371-382 (1989); coussens, P. M., et al., J. Virol. 62:2373-2379 (1988); Isfort, R. J., et al., J. Virol. 59:411-419 (1986); Isfort, R. J., et al., J. Virol. 57:464-474 (1986); and Sithole, I., et al., J. Virol. 62:4270-4279 (1988)). This is despite findings of three MDV gD, gI and gE homologs of the present invention and two additional glycoprotein homologs (gB and gH; Buckmaster, A. E., et al., J. Gen. Virol. 69:2033-2042 (1988); and Ross, L. J. N., et al., J. Gen. Virol. 70:1789-1804 (1989)). While immune chicken sera (ICS) from naturally infected birds is likely to react with many, if not all, MDV-encoded surface antigens, this complex polyclonal sera would is only be useful to the extent that antigen expression/processing in semi-productive cell cultures resembles that in feather follicle epithelial cells. Northern blot analysis using MDV gD-specific probes suggests that MDV gD mRNA is either not expressed or poorly expressed in DEF cells at a time when extensive cytopathic effects are observed (data not shown). In light of the fact that VZV lacks a gD homolog and is strongly cell-associated, it will be interesting to see whether the block in MDV virion formation in primary avian fibroblast cells is found to correlate with lack of expression (in these cells) of a glycoprotein, such as gD, and/or some other S component gene(s). 
     Because the protection against MD conferred by attenuated MDV strains (serotype 2) or HVT (serotype 3) appears to have an immunological basis, there is considerable interest in identifying common antigens. In view of this invention identifying seven MDV U S  homologs to U S  genes of HSV (the latter of which is clearly less related to MDV than HVT is), it would be surprising if the previous report showing lack of homology between MDV-HVT U S  regions (Igarashi, T., et al., Virology 157:351-358 (1987)) were proven correct. Such negative results may reflect the limitations regarding homology estimates based on hybridization, rather than sequence analysis studies. 
     Example 2 shows the molecular cloning of a construct containing the DNA encoding the complete MDV US7 (gI) and part of MDV US8 (gE) genes. As can be seen, this is accomplished using segments of DNA spanning the gI and part of the gE coding region. 
     EXAMPLE 2 
     MOLECULAR CLONING OF A CONSTRUCT CONTAINING THE DNA ENCODING THE COMPLETE MDV US7 (gI) AND PART OF MDV US8 (gE) 
     Construction of a recombinant clone (pKS-MDgI1.59) containing the complete MDV US7 (gI) coding sequence and a portion of the MDV US8 (gE) coding sequence requires two preexisting MDV clones, pKS-MDgD1.75 and p19P1 (FIG. 5). pKS-MDgD1.75 is a recombinant plasmid containing the 1.75 kbp NcoI-SstII subfragment of MDV EcoR1-I legated into the SmaI-Sst II site of the cloning vector, pBluescript KS-. This clone contains the complete MDV US6 (gD) coding sequence and additional sequences at the 3&#39; end which code for the first 39 amino acids (aa) of MDV gI. p19P1 is a recombinant plasmid containing the 1.5 kbp BamHI-P 1 , subfragment of MDV cloned into the unique BamHI site of pUC19. This clone contains the entire MDV gI coding sequence, except for the first 9 aa of its signal sequence. In addition, at the 3&#39; end, p19P1 contains the first 104 aa of the MDV US8 (gE) coding region. 
     To generate pKS-MDgI1.59, pKS-MgD1.75 is first cut with Hinc II, which cuts once in the multiple cloning site of the pBluescript vector and once about 180 bp upstream of the insert&#39;s Sst II terminus. This results in two fragments: one fragment (1.6 kbp) consists primarily of insert sequences encoding MDV US6(gD); the larger fragment (3.1 kbp) consists of pBluescript vector sequences, in addition to about 180 bp which encode the N-terminus of MDV gI. The 3.1 kb fragment is gel purified and self-ligated by way of the two Hinc II ends. The resulting recombinant plasmid, pKS-MDgI0.18, is then cut with Sst I (in the multiple cloning site, just downstream of the SstII site). Prior to subsequent digestion with SstII, the cohesive Sst I ends is made blunt-ended with T4 DNA polymerase. The resulting 3.1 kbp Sst II-Sst I(blunt) fragment of pMDgI0.18 is gel purified and used in the final ligation step to create pKS-MDgI1.59. While the enzymatic manipulations of pKS-MDgD1.75  and pKS-MDgI0.18 are taking place, p19P1 is cut with Hind III, which cuts just downstream of the partial MDV US8 (gE) coding sequence in the multiple cloning site of pUC19. Prior to digestion with SstII, the cohesive Hind III ends is made blunt-ended using Klenow fragment. The smaller Sst II-Hind III(blunt) fragment (1.4 kbp) contains a majority of the MDV US7 (gI) coding sequence, in addition to 312 nucleotides at the 3&#39; end which code for the 5&#39; end of MDV gE. This 1.4 kbp Sst II-Hind III(blunt) fragment is gel purified and ligated to the 3.1 kbp Sst II-Sst I(blunt) fragment of pKS-MDgD0.18. The resulting recombinant, pKS-MDgI1.59, contains the complete coding sequence for MDV gI and a portion of the N-terminal gE coding sequence. Digestion of pKS-MDgI1.59 with KpnI yields two fragments; the smaller 1.15 kbp fragment contains the complete coding sequence for MDV gI. 
     Plasmid pKS-MDgI1.59, containing the DNA sequences encoding the complete glycoprotein MDV gI and part of the glycoprotein MDV gE, is on deposit at Michigan State University. 
     Example 3 shows the strategy for isolating a construct containing complete MDV US8 (gE) gene. As can be seen, this is accomplished by using a segment of DNA encoding the N terminal end of the gE gene as a radiolabeled probe to screen genomic and/or cDNA libraries. 
     EXAMPLE 3 
     STRATEGY FOR ISOLATING A CONSTRUCT CONTAINING COMPLETE MDV US8 (gE) GENE 
     Double digestion of pKS-MDgI1.59 with KpnI and BamHI yields a 425 bp KpnI-BamHI subfragment containing the coding sequences for the N-terminal 104 amino acids of MDV gE. This subfragment is radiolabelled and used as a probe to screen both genomic and CDNA libraries of MDV in order to locate clones containing the complete MDV gE gene. Positively hybridizing clones are further characterized and sequenced in order to better define the complete structure of this gene. 
     A summary of MDV data is shown in Table 1, with location of ORFs, predicted polyadenylation signals utilized, translational context nucleotides, lengths, relative molecular sized and isoelectric points of predicted translation products. 
     
                                           TABLE 1                                 
__________________________________________________________________________
           Predicted                                                      
           Poly-                                                          
                -3, +4 ATG.sup.a                                          
                            Predicted                                     
                                  Pre-                                    
     ORF                                                                  
        ORF                                                               
           adenyla-                                                       
                Context     Molecular                                     
                                  dicted                                  
Name Start                                                                
        End                                                               
           tion Site                                                      
                Nucleotides                                               
                      Length (aa)                                         
                            Size (kDa)                                    
                                  pI.sup.b                                
__________________________________________________________________________
US1   248                                                                 
         784                                                              
           1777 A,A   179   20.4  6.5                                     
US10 1077                                                                 
        1715                                                              
           1777 G,G   213   23.6  8.2                                     
SORF1                                                                     
     2884                                                                 
        1832                                                              
           1790 A,A   351   40.6  8.2                                     
US2  3923                                                                 
        3114                                                              
           1790 A,G   270   29.7  7.6                                     
US3  4062                                                                 
        5240                                                              
           5394 A,G   393   43.8  6.1                                     
SORF2                                                                     
     5353                                                                 
        5793                                                              
           5904 C,G   147   16.7  9.8                                     
US6  5964                                                                 
        7172                                                              
           c    G,G   403   42.6.sup.d                                    
                                  10.3.sup.d                              
US7  7282                                                                 
        8346                                                              
           c    G,T   355   38.3.sup.d                                    
                                  6.7.sup.d                               
US8  8488                                                                 
        e  c    A,T   e     e     e                                       
__________________________________________________________________________
 .sup.a Nucleotides listed relative to -3, +4 positions, respectively;    
 numbering begins with the A of the ATG (AUG) codon as position +1;       
 nucleotides 5&#39; to that site are assigned negative numbers.               
 .sup.b Calculated using the GCG program, ISOELECTRIC.                    
 .sup.c Likely to use undefined site just downstream of US8 termination   
 codon.                                                                   
 .sup.d Based on sequences that follow the predicted signal peptide       
 cleavage site.                                                           
 .sup.e Incomplete ORF.                                                   
 
    
     
                                           TABLE 2                                 
__________________________________________________________________________
US1                   US10                                                
Virus MDV HSV-1                                                           
              VZV EHV-4                                                   
                      MDV HSV-1                                           
                              VZV EHV-4                                   
__________________________________________________________________________
MDV   891 47/26                                                           
              43/27                                                       
                  48/30                                                   
                      1071                                                
                          45/24                                           
                              40/24                                       
                                  45/29                                   
HSV-1 101 2047                                                            
              49/29                                                       
                  50/29                                                   
                      134 1617                                            
                              49/27                                       
                                  49/27                                   
VZV   160 119 1378                                                        
                  54/36                                                   
                      147 123 978 55/32                                   
EHV-4 208 150 359 1308                                                    
                      251 180 191 1312                                    
Length (aa)                                                               
      179 420 278 273 213 312 180 259                                     
__________________________________________________________________________
US2           US3         US6                                             
Virus MDV HSV-1                                                           
              MDV HSV-1                                                   
                      VZV MDV HSV-1                                       
                                  PRV                                     
__________________________________________________________________________
MDV   1421                                                                
          51/33                                                           
              1931                                                        
                  56/38                                                   
                      54/33                                               
                          1965                                            
                              43/22                                       
                                  44/24                                   
HSV-1 335 1554                                                            
              611 2409                                                    
                      57/41                                               
                          211 1999                                        
                                  47/27                                   
VZV   --  --  616 717 1960                                                
                          --  --  --                                      
PRV   ?   ?   ?   ?   ?   279 294 2116                                    
Length (aa)                                                               
      270 291 393 481 393 403 394 402                                     
__________________________________________________________________________
US7                   US8*                                                
Virus MDV HSV-1                                                           
              VZV PRV MDV HSV-1                                           
                              VZV PRV                                     
__________________________________________________________________________
MDV   1816                                                                
          39/22                                                           
              46/23                                                       
                  43/25                                                   
                      538 36/17                                           
                              44/29                                       
                                  47/30                                   
HSV-1 145 1880                                                            
              43/24                                                       
                  47/25                                                   
                       56 583 40/19                                       
                                  44/23                                   
VZV   228 234 1705                                                        
                  41/26                                                   
                      109  30 624 47/30                                   
EHV-4 184 188 198 1652                                                    
                      103  59 103 593                                     
Length (aa)                                                               
      355 390 354 350 104 118 126 125                                     
__________________________________________________________________________
 .sup.a FASTA scores represented by single numbers in lower left of boxes.
 .sup.b Similarity/identity percentages are derived from pairwise GAP     
 comparisons and are located in upper right of boxes.                     
 .sup.-- no VZV homolog.                                                  
 .sup.? PRV homologs identified (M. M. van Zijl, J. M. L. vander Gulden, A
 L. J. Gielkens and A. J. M. Berns, Abstr. 14th Int. Herpesvirus Workshop,
 p. 5, 1989), but published sequences not available                       
 *regions compared limited to those aligned with the partial MDV US8      
 sequence                                                                 
 
    
     
                       TABLE 3                                                     
______________________________________                                    
      Predicted   Trans-    Cyto-                                         
      Signal Peptide                                                      
                  membrane  plasmic                                       
                                   N-glycosylation                        
Name  Cleavage Site                                                       
                  Domain    Domain Sites                                  
______________________________________                                    
US6   G.sub.30 -D.sub.31                                                  
                  358-374   375-403                                       
                                    87,138,230,306                        
US7   S.sub.18 -I.sub.19                                                  
                  269-288   289-355                                       
                                   147,167,210,245,                       
                                   253,317                                
US8   T.sub.18 -A.sub.19                                                  
                  a         a      a                                      
 .sup.a incomplete ORF                                                    
 
    
     Index of definition of letters in FIG. 2. Table 4 showing the amino acids with both their single letter and three letter symbols. 
     
                       TABLE 4                                                     
______________________________________                                    
A            Ala         Alanine                                          
C            Cys         Cysteine                                         
D            Asp         Aspartic Acid                                    
E            Glu         Glutamic Acid                                    
F            Phe         Phenylalanine                                    
G            Gly         Glycine                                          
H            His         Histidine                                        
I            Ile         Isoleucine                                       
K            Lys         Lysine                                           
L            Leu         Leucine                                          
M            Met         Methionine                                       
N            Asn         Asparagine                                       
P            Pro         Proline                                          
Q            Gln         Glutamine                                        
R            Arg         Arginine                                         
S            Ser         Serine                                           
T            Thr         Threonine                                        
V            Val         Valine                                           
W            Trp         Tryptophan                                       
Y            Tyr         Tyrosine                                         
______________________________________                                    
 
    
     ATCC Deposit 
     The gene for MDV US6 (MDV gD) has been deposited in a plasmid (phagemid) PKS-MDgD1.75, as ATCC 40855, with The American Type Culture Collection, Rockville, Md. 20852, USA.