Dennis J. O’Callaghan

Characterization of the transactivation domain of the equine herpesvirus type 1 immediate-early protein.

Equine herpesvirus type 1 (EHV-1) possesses a sole diploid immediate early gene (IE) that encodes a major regulatory protein of 1487 amino acids capable of modulating gene expression from both early and late promoters and also of trans-repressing its own promoter. Using a series of GAL-4-IE fusion constructs, we previously demonstrated that the minimal transactivation domain (TAD) of the IE protein maps within amino acids 3-89. Additional studies revealed that that the carboxyl terminus of the IE protein may be required for full transactivation activity in vitro. Analyses of the minimal TAD revealed the presence of 13 acidic amino acids and six basic residues giving the TAD region a net negative charge of -7. In addition, there are conserved hydrophobic residues (Leu(12) and Phe(15)) that may be critical for transactivation function. To identify residues essential for IE transactivation and to ascertain if the overall net negative charge of the TAD or the position of specific hydrophobic residues within the IE TAD are critical for the transactivation function, plasmids expressing mutant forms of the TAD were generated using specifically designed mutagenic oligonucleotides and PCR mutagenesis. Mutagenized TADs in which the acidic and hydrophobic amino acid residues were replaced, singly and in combination, with polar, uncharged amino acids were cloned into a GAL-4/CAT reporter expression system and assayed in transient transfection assays. To determine if the carboxyl terminus is necessary for full transactivation activity, a series of constructs that express forms of the IE protein-containing deletions within this region were generated and assayed for transactivation function in transient transfection assays. These assays demonstrated that mutation of any acidic residue, either singly or in combination, or deletion of the carboxyl terminus of the IE protein resulted in a severe impairment of transactivation activity. These results show that both acidic and hydrophobic residues within the IE TAD are critical for transactivation function and that the carboxyl terminus of the IE protein is required for full transactivation activity.

The defective interfering particles of equine herpesvirus 1 encode an ICP22/ICP27 hybrid protein that alters viral gene regulation

The genomes of equine herpesvirus 1 (EHV-1) defective interfering (DI) particles that mediate persistent infection were shown to encode a unique hybrid open reading frame composed of sequences that encode the 196 N-terminal amino acids of ICP22 linked in-frame to the C-terminal 68 amino acids of ICP27. Previous studies demonstrated that this hybrid gene, designated as ICP22/ICP27. was expressed abundantly at both the mRNA and the protein levels in DI particle-enriched infections, but not in standard EHV-1 infection (Chen et al., 1996 J. Virol. 70, 313-320). Since the ICP22/ICP27 hybrid protein contains portions of two EHV-1 early regulatory proteins, its effect on EHV-1 gene regulation was investigated. In EHV-1-infected cells, the ICP22/ICP27 hybrid protein expressed from plasmid vectors significantly reduced expression of a reporter gene under the control of the EHV-1 immediate-early (IE) gene promoter and early gene promoter, such as the viral ICP27 gene. In uninfected cells, the ICP22/ICP27 hybrid protein moderately down-regulated the IE and ICP22 promoters, up-regulated late gene promoters such as IR5, and altered the regulatory function of the IE and 1CP22 proteins in co-transfected cells. These results demonstrated that DI particles might alter viral gene regulation by expression of a unique hybrid gene encoded on the DI particle genome.

The early UL31 gene of equine herpesvirus 1 encodes a single-stranded DNA-binding protein that has a nuclear localization signal sequence at the C-terminus

The amino acid sequence of the UL31 protein (UL31P) of equine herpesvirus 1 (EHV-1) has homology to that of the ICP8 of herpes simplex virus type 1 (HSV-1). Here we show that the UL31 gene is synergistically trans-activated by the IEP and the UL5P (EICP27). Detection of the UL31 RNA transcript and the UL31P in EHV-1-infected cells at 6h post-infection (hpi) as well as metabolic inhibition assays indicated that UL31 is an early gene. The UL31P preferentially bound to single-stranded DNA over double-stranded DNA in gel shift assays. Subcellular localization of the green fluorescent protein (GFP)-UL31 fusion proteins revealed that the C-terminal 32 amino acid residues of the UL31P are responsible for the nuclear localization. These findings may contribute to defining the role of the UL31P single-stranded DNA-binding protein in EHV-1 DNA replication.

Development of a bacterial artificial chromosome (BAC) recombineering procedure using galK-untranslated region (UTR) for the mutation of diploid genes.

Bacterial artificial chromosome (BAC) recombineering using galK selection allows DNA cloned in Escherichia coli to be modified without introducing an unwanted selectable marker at the modification site. Genomes of some herpesviruses have a pair of inverted repeat sequences that makes it very difficult to introduce mutations into diploid (duplicate) genes using the galK selection method. To mutate diploid genes, we developed a galK-UTR BAC recombineering procedure that blocks one copy of the target diploid gene by insertion of a galK untranslated region (UTR), which enables the simple mutation of the other copy. The blocked copy can then be replaced with an UTR-specific primer pair. The IR2 gene of equine herpesvirus 1 (EHV-1) maps within both the internal (IR) and terminal repeat (TR) of the genomic short region and is expressed at low levels because its promoter is TATA-less. Both IR2 promoters in EHV-1 BAC were replaced with a mutant IR2 promoter containing three Sp1-binding motifs and a consensus TATA box by galK-UTR BAC recombineering. The expression level of the IR2 protein controlled by the modified promoter increased approximately 4-fold as compared to that of wild-type EHV-1. The galK-UTR method will provide a useful tool in studies of herpesviruses.

The EHV-1 UL4 protein that tempers viral gene expression interacts with cellular transcription factors

The UL4 gene is conserved within the genome of defective interfering particles of equine herpesvirus type 1 (EHV-1) that mediate persistent infection. Here, we show that the UL4 protein inhibits EHV-1 reporter gene expression by decreasing the level of transcribed mRNA. The UL4 protein did not bind any gene class of EHV-1 promoters in electromobility or chromatin immunoprecipitation assays, but directly interacted with the TATA box-binding protein (TBP) and the carboxy-terminal domain of RNA polymerase II both in vitro (GST-pulldown assays) and in infected cells (coimmunoprecipitation analyses). Microarray analyses of the expression of the 78 EHV-1 genes revealed that viral late genes important for virion assembly displayed enhanced expression in cells infected with UL4-null virus as compared to wild-type or UL4-restored EHV-1. Quantitative PCR analyses showed that viral DNA replication was not retarded in cells infected with the UL4-null virus as compared to wild-type EHV-1.

Deletion of the UL4 gene sequence of equine herpesvirus 1 precludes the generation of defective interfering particles

Serial, high multiplicity passage of equine herpesvirus 1 (EHV-1) leads to the generation of defective interfering particles (DIP). EHV-1 DIP inhibit and interfere with the replication of standard EHV-1, establishing a state of persistent infection. These DIP package severely truncated and rearranged forms of the standard viral genome. Contained within the DIP genome are only three genes: UL3, UL4, and a unique hybrid gene (Hyb). The hybrid gene forms through a recombination event that fuses portions of the early regulatory IR4 and UL5 genes and is essential for DIP-mediated interference. The UL4 gene is an early gene dispensable for lytic replication and inhibits viral and cellular gene expression. However, the contribution of the UL4 gene during DIP-mediated persistent infection is unknown. Here, we describe the generation of a completely deleted UL4 virus and its use to investigate the role of the UL4 gene in the generation of the defective genome. Deletion of the UL4 gene resulted in delayed virus growth at late times post-infection. Cells infected with a mutant EHV-1 that lacked expression of the UL4 protein due to an inserted stop codon in the UL4 gene produced defective particles, while cells infected with a mutant EHV-1 that had the complete UL4 gene sequence deleted were unable to produce DIP. These data suggest that the UL4 gene sequence, but not the UL4 protein, is critical for the generation of defective interfering particles.

Comparative Genomic Sequencing and Pathogenic Properties of Equine Herpesvirus 1 KyA and RacL11

Equine herpesvirus 1 (EHV-1) is a major pathogen affecting equines worldwide. The virus causes respiratory disease, abortion, and, in some cases, neurological disease. EHV-1 Kentucky A (KyA) is attenuated in the mouse and equine, whereas wild-type pathogenic strain RacL11 induces severe inflammatory infiltration of the lung, causing infected mice to succumb. The complete DNA sequencing of the KyA genome revealed that genes UL17 (ORF17), US6 (ORF73; gI), US7 (ORF74; gE), and US8 (ORF75; 10 K) are deleted as compared to the RacL11 and Ab4 genomes. In-frame deletions in the US1 (ORF68), US4 (ORF71; gp2), and UL63 (ORF63; EICP0) genes and point mutations in 14 different open reading frames (ORFs) were detected in the KyA genome. Interestingly, UL1 (ORF1) and UL2 (ORF2) were deleted in both KyA and RacL11. Our previous studies showed that EHV-1 glycoproteins gI, gE, and full-length gp2 contribute to the pathogenesis of the RacL11 strain. The confirmation of these gene deletions in KyA suggests their contribution to the attenuation of this virus. The growth kinetics results revealed that KyA replicates to high titers in cell culture as compared to RacL11 and Ab4, indicating that the above genomic deletions and mutations in KyA do not have an inhibitory effect on KyA replication in cells of mouse, rabbit, equine, or human origin. Studies of EHV-1 pathogenesis in CBA mice showed that KyA is attenuated whereas mice infected with RacL11 succumbed by 3–6 days post-infection, which is consistent with our previous results.

Full trans-activation mediated by the immediate-early protein of equine herpesvirus 1 requires a consensus TATA box, but not its cognate binding sequence.

The immediate-early protein (IEP) of equine herpesvirus 1 (EHV-1) has extensive homology to the IEP of alphaherpesviruses and possesses domains essential for trans-activation, including an acidic trans-activation domain (TAD) and binding domains for DNA, TFIIB, and TBP. Our data showed that the IEP directly interacted with transcription factor TFIIA, which is known to stabilize the binding of TBP and TFIID to the TATA box of core promoters. When the TATA box of the EICP0 promoter was mutated to a nonfunctional TATA box, IEP-mediated trans-activation was reduced from 22-fold to 7-fold. The IEP trans-activated the viral promoters in a TATA motif-dependent manner. Our previous data showed that the IEP is able to repress its own promoter when the IEP-binding sequence (IEBS) is located within 26-bp from the TATA box. When the IEBS was located at 100 bp upstream of the TATA box, IEP-mediated trans-activation was very similar to that of the minimal IE(nt -89 to +73) promoter lacking the IEBS. As the distance from the IEBS to the TATA box decreased, IEP-mediated trans-activation progressively decreased, indicating that the IEBS located within 100 bp from the TATA box sequence functions as a distance-dependent repressive element. These results indicated that IEP-mediated full trans-activation requires a consensus TATA box of core promoters, but not its binding to the cognate sequence (IEBS).