Cloning and sequence analysis of a Marek's disease virus origin binding protein (OBP) reveals strict conservation of structural motifs among OBPs of divergent alphaherpesviruses

The herpes simplex virus type 1 DNA polymerase processivity factor, UL42, does not alter the catalytic activity of the UL9 origin-binding protein but facilitates its loading onto DNA

The herpes simplex virus type 1 UL42 DNA polymerase processivity factor interacts physically with UL9 and enhances its ability to unwind short, partially duplex DNA. In this report, ATP hydrolysis during translocation of UL9 on single-stranded (ss) or partially duplex DNA was examined in the presence and absence of UL42 to determine the effect of UL42 on the catalytic function of UL9. Our studies reveal that a homodimer of UL9 is sufficient for DNA translocation coupled to ATP hydrolysis, and the steady-state ATPase catalytic rate was greater on partially duplex DNA than on ss DNA in the presence or absence of UL42. Although UL42 protein increased the steady-state rate for ATP hydrolysis by UL9 during translocation on either partially duplex or ss DNA, UL42 had no significant effect on the intrinsic ATPase activity of UL9. UL42 also had no effect on the catalytic rate of ATP hydrolysis when UL9 was not limiting but enhanced the steady-state ATPase rate at only subsaturating UL9 concentrations. At subsaturating UL9 to DNA ratios, stoichiometric concentrations of UL42 were shown to increase the amount of UL9 bound to ss DNA at equilibrium. These data support a model whereby UL42 increases the ability of UL9 to load onto DNA, thus increasing its ability to assemble into a functional complex capable of unwinding duplex DNA.

Characterization of the chromosomal binding sites and dimerization partners of the viral oncoprotein Meq in Marek's disease virus-transformed T cells

Marek's disease virus (MDV) is an acute transforming alphaherpesvirus that causes T-cell lymphomas in chickens. We previously reported the identification of a putative oncogene, meq, that is encoded only by the oncogenic serotype of MDV. The gene product, Meq, is a latent protein that is consistently expressed in MDV-transformed lymphoblastoid cells and tumor cells. Meq has a bZIP (basic leucine zipper) structure resembling the family of Jun/Fos. The mechanism whereby Meq transforms T cells remains poorly understood. In this study, we explored the properties of Meq as a transcriptional factor. We analyzed Meq's dimerization partners and its target genes in MSB-1, an MDV-transformed T-cell line. By using in vitro assays, we first demonstrated Meq's potential to dimerize with a variety of bZIP proteins. We then identified c-Jun as the primary dimerization partner of Meq. Both are found to be colocalized in the nucleus and corecruited to promoters with AP-1 sequences. By using chromatin immunoprecipitation (ChIP), we scanned the entire MDV genome for Meq binding sites and found three regions that were enriched with Meq binding: the MDV lytic replication origin, the promoter for Meq, and the promoter for ICP4. Transactivation assays using the above promoters showed that Meq/Meq homodimers exhibited repression activity, whereas Meq/Jun heterodimers showed activation. Finally, we were able to show by ChIP that Meq is recruited to the interleukin-2 promoter in a region encompassing an AP-1 site. Thus, in addition to providing general knowledge about the transcriptional properties of Meq, our studies revealed for the first time the ability of Meq to interact with the latent MDV and host genomes. Our data suggest, therefore, a role for Meq in viral genome regulation during latency, in addition to its putative causal role in T-cell transformation.

Functional characterization of Marek's disease virus (MDV) origin-binding protein (OBP): analysis of its origin-binding properties

In previous studies, we identified a Marek's disease virus (MDV) origin-binding protein (OBP) gene that is highly homologous to the herpes simplex virus type 1 UL9 gene that encodes an OBP and functions as an initiator protein for viral DNA replication. In this study, a protein of 95 kDa was produced in coupled in vitro transcription-translation reaction with the plasmid containing the wild type MDV OBP gene. The in vitro synthesized protein was detected by immunoprecipitation with a penta-histidine specific monoclonal antibody. Further characterization of MDV OBP was accomplished using electrophoretic mobility shift assay (EMSA) with the in vitro expressed MDV OBP using a double-stranded (ds) 26-mer oligonucleotide as the probe, which was designed from the putative MDV OBP binding site present in the serotype 1 or 2 MDV replication origin. The EMSA results indicated that MDV OBP could form a protein-DNA complex with the ds 26-mer oligonucleotide designed from serotype 1 or 2 replication origin. A series of 26-mer oligonucleotides with two-base-pair (bp) substitution across the putative MDV OBP binding site were used in competitive EMSA to determine the recognition sequence for the MDV OBP. The results demonstrated that the recognition sequence for MDV OBP was the TTCGCACC that is a subset of a 9-bp element (CGTTCGCAC) conserved in the replication origins of alphaherpesviruses. Furthermore, the results of EMSA with a series of deletion mutants from the N-terminus of MDV OBP indicated that the origin-binding domain was located at the amino acids region 528 to 841 of the wild-type MDV OBP. Taken together, our results suggest that the MDV OBP gene encodes an OBP of MDV.

Differences in DNA binding specificity among Roseolovirus origin binding proteins

The Roseolovirus genus of the Betaherpesvirinae consists of the very closely related viruses, human herpesvirus 6 variants A and B (HHV-6A and HHV-6B) plus the somewhat more distantly related human herpesvirus 7 (HHV-7). The roseoloviruses each encode a homolog of the alphaherpesvirus origin binding protein (OBP) which is required for lytic DNA replication. In contrast, members of the other betaherpesvirus genera, the cytomegaloviruses, initiate DNA replication by a different mechanism. To better understand the basis of roseolovirus OBP sequence specificity, we investigated their ability to recognize each other's binding sites. HHV-6A OBP (OBP(H6A)) and HHV-6B OBP (OBP(H6B)) each bind to both of the HHV-7 OBP sites (OBP-1 and OBP-2) with similar strengths, which are also similar to their nearly equivalent interactions with their own sites. In contrast, HHV-7 OBP (OBP(H7)) had a gradient of binding preferences: HHV-7 OBP-2 > HHV-6 OBP-2 > HHV-7 OBP-1 > HHV-6 OBP-1. Thus, the roseolovirus OBPs are not equally reciprocal in their recognition of each other's OBP sites, suggesting that the sequence requirements for the interaction of OBPH7 at the OBP sites in its cognate oriLyt differ from those of OBPH6A and OBPH6B.

The genome of a very virulent Marek's disease virus

Here we present the first complete genomic sequence, with analysis, of a very virulent strain of Marek's disease virus serotype 1 (MDV1), Md5. The genome is 177,874 bp and is predicted to encode 103 proteins. MDV1 is colinear with the prototypic alphaherpesvirus herpes simplex virus type 1 (HSV-1) within the unique long (UL) region, and it is most similar at the amino acid level to MDV2, herpesvirus of turkeys (HVT), and nonavian herpesviruses equine herpesviruses 1 and 4. MDV1 encodes 55 HSV-1 UL homologues together with 6 additional UL proteins that are absent in nonavian herpesviruses. The unique short (US) region is colinear with and has greater than 99% nucleotide identity to that of MDV1 strain GA; however, an extra nucleotide sequence at the Md5 US/short terminal repeat boundary results in a shorter US region and the presence of a second gene (encoding MDV097) similar to the SORF2 gene. MD5, like HVT, encodes an ICP4 homologue that contains a 900-amino-acid amino-terminal extension not found in other herpesviruses. Putative virulence and host range gene products include the oncoprotein MEQ, oncogenicity-associated phosphoproteins pp38 and pp24, a lipase homologue, a CxC chemokine, and unique proteins of unknown function MDV087 and MDV097 (SORF2 homologues) and MDV093 (SORF4). Consistent with its virulent phenotype, Md5 contains only two copies of the 132-bp repeat which has previously been associated with viral attenuation and loss of oncogenicity.

DNA sequence of the UL6 to UL20 genes of infectious laryngotracheitis virus and characterization of the UL10 gene product as a nonglycosylated and nonessential virion protein

The 24 kbp KpnI restriction fragment A from the unique long genome region of infectious laryngotracheitis virus (ILTV, gallid herpesvirus-1) has been sequenced. The analysed region contains 14 open reading frames sharing homology with conserved alphaherpesvirus genes. Arrangement of the UL6 to UL20 homologues of ILTV is almost identical to that found in the herpes simplex virus type 1 genome. As in other herpesviruses the UL15 gene consists of two exons and is expressed from a spliced mRNA. However, the UL16 gene, which is usually localized within the intron sequence of UL15, is not conserved at this position of the ILTV genome. Another unique feature is the absence of any putative N-glycosylation motifs within the deduced ILTV UL10 gene product, which is the homologue of the conserved herpesvirus glycoprotein M. After preparation of a monospecific antiserum, two distinct UL10 proteins with apparent molecular masses of 36 and 31 kDa were identified in ILTV-infected cells as well as in purified virions. None of these UL10 gene products is modified by N- or O-linked glycosylation. Isolation of a green fluorescent protein-expressing UL10 deletion mutant of ILTV revealed that this gene is not required for virus replication in cell culture.