Immediate early protein of pseudorabies virus is a general transactivator but stimulates only suboptimally utilized promoters. A clue to specificity?

The TATA motif specifies the differential activation of minimal promoters by varicella zoster virus immediate-early regulatory protein IE62

The immediate-early IE62 protein of varicella zoster virus is an acidic transcriptional activator capable of up-regulating many viral and cellular promoters with varying efficiencies. We demonstrate that, in the context of a minimal promoter, a TATA element is both sufficient and essential for IE62-mediated transcriptional activation. Differential levels of activation by IE62 in this context were conferred by a panel of naturally occurring sequence variations within the TATA motif itself. TATA motif-specific, differential induction was not obtained when the IE62 acidic activation domain was targeted as a GAL4 fusion protein to the same panel. The prototype acidic transactivator, VP16 of herpes simplex virus, failed to discriminate between these different TATA motifs when they were placed into an appropriate responsive promoter context. Nonetheless, a chimeric IE62 polypeptide substituted with the VP16 activation domain retained the ability to differentially modulate minimal promoters with various TATA motifs. Taken together with its binding to TATA box-binding protein (TBP) and transcription factor IIB in vitro, we suggest that IE62 has the unusual ability to achieve differential levels of transcriptional activation through different TATA motifs, which may be accomplished either directly or indirectly by recognizing conformational variations in DNA-bound TBP, TBP-transcription factor IIA/B, or TBP-TATA-associated factor complexes.

Promoter activity of sequence located upstream of the pseudorabies virus early protein 0 gene

Promoter activity of the 5'-flanking region of the pseudorabies virus (PRV) early protein 0 (EP0) gene was analysed by transient transfection assays employing chloramphenicol acetyl transferase (CAT) reporter constructs. We identified a 213 bp segment of the viral genome that was capable of efficiently driving expression of the EPO gene and a linked reporter gene upon transient transfection into Vero cells. This segment lacked the typical TATA element, and possessed an initiator element and the putative binding sites for the transcription factor Sp1 and immediate-early protein IE180, a strong transactivator of PRV. By analysing 5'-deletion mutants of the segment, a 48 bp segment (from nucleotide positions -65 to -17), which possessed three Sp1 binding sites, was identified to be critical for the promoter activity. Cotransfection of Vero cells with the mutant constructs and an IE180 expression plasmid resulted in transactivation of only those constructs in which the Sp1 sites were present. These results indicate that the EP0 gene may be transcribed from the TATA-less promoter that responds to Sp1.

Strong transcriptional activators isolated from viral DNA by the 'activator trap', a novel selection system in mammalian cells

Transcription factors often contain activation domains that interact with the basic transcription machinery. We have developed a functional screening strategy in mammalian cells to selectively isolate activation domains from a library of random DNA inserts. For this, sonicated DNA fragments are cloned next to the DNA binding domain of GAL4 factor in a plasmid that also contains the SV40 origin of replication. Pools of fusion protein clones are transfected into CV-1-5GT monkey cells containing an SV40 T antigen gene under the control of a promoter with GAL4 binding sites. Plasmids that express functional transactivating fusion proteins activate the T antigen gene, thus promoting selective amplification of the plasmid in the mammalian host cell line. Using this method, we were able to select strong enhancer-type activation domains from the immediate early regions of two herpesviruses, namely pseudorabies virus and bovine herpesvirus 1. In both cases, the activation domains selected were homologues of the ICP4 regulatory protein of herpes simplex virus. The activation domain from pseudorabies virus is four times stronger than the activation domain of herpes simplex virus protein VP16 (Vmw65), making it the strongest activation domain characterized so far. This activator trap method should be useful for precisely localizing activation domain(s) in known factors, or to identify mammalian transcriptional adaptors that do not bind DNA and which may escape conventional detection methods.

The DNA binding domain of the varicella-zoster virus gene 62 protein interacts with multiple sequences which are similar to the binding site of the related protein of herpes simplex virus type 1

Varicella-zoster virus gene 62 encodes a protein with predicted Mr of 140,000D (VZV 140k) that shares extensive predicted amino acid sequence homology with the major immediate early (IE) transcriptional regulator protein of herpes simplex virus type 1 (HSV-1) Vmw175. The integrity of highly conserved region 2 is essential for the DNA binding and transcriptional regulatory functions of Vmw175. Similarly, an insertion mutation in region 2 (codons 468-641) of 140k eliminates the transcriptional repression and activation functions of this protein. We have expressed a fragment of 140k which encompasses region 2 as a non-fusion polypeptide in bacteria. This 140k DNA binding domain peptide (codons 417-646) binds to numerous DNA sequences throughout the VZV gene 62 promoter region. It induces multiple regions of protection from DNase I digestion, flanked by sites of DNase I hypersensitivity. Several of the sites recognized can be considered to be divergent forms of the consensus sequence which is recognized by Vmw175. However, by use of a panel of mutagenized probe fragments, we found that the 140k DNA binding domain was less sequence-specific than Vmw175 in its interactions with DNA. Consistent with this, the homologous Vmw175 DNA binding domain, and also intact Vmw175, recognize the gene 62 binding sites much less efficiently than the 140k DNA binding domain. Also in contrast to the situation with Vmw175, the 140k DNA binding domain failed to induce DNA bending when occupying the binding sites in its own promoter. Deletion analysis has mapped the minimal DNA binding domain of the VZV 140k protein, as measured in gel retardation analysis, to lie within residues 472 to 633. The differences in binding characteristics of the DNA binding domains of the homologous VZV 140k and HSV-1 Vmw175 IE proteins may account for the subtle differences in their regulatory activities in transfection assays and during virus growth in tissue culture.

Substitution of a TATA box from a herpes simplex virus late gene in the viral thymidine kinase promoter alters ICP4 inducibility but not temporal expression

The role of cis-acting promoter elements associated with herpes simplex virus type 1 (HSV-1) early and late genes was evaluated during productive infection with regard to activation of gene expression by the HSV-1 transactivator ICP4 and control of temporal regulation. A set of recombinant viruses was constructed such that expression of an HSV-1 early gene, thymidine kinase (tk), was placed under the control of either the tk TATA box or the TATA box from the late gene, glycoprotein C (gC), in the presence or absence of the upstream Sp1 and CCAAT sites normally found in the tk promoter. The presence of Sp1 sites in the promoter or replacement of the tk TATA box with the gC TATA box resulted in a decreased activation of tk mRNA expression by ICP4. Substitution of the A + T-rich region from the gC TATA box in the context of the remainder of the surrounding tk sequences resulted in a promoter that bound recombinant TATA-binding protein (TBP) better at lower concentrations than the wild-type tk promoter did. These results indicate that tk promoters that are better able to utilize TBP are less responsive to ICP4 activation and suggest that activation by ICP4 involves the general transcription factors that interact with TBP or TBP itself. Additionally, all of the viruses expressed tk at early times postinfection, indicating that cis-acting promoter elements that control the level of expression of HSV-1 early and late genes do not determine temporal regulation.

The three major immediate-early transcripts of bovine herpesvirus 1 arise from two divergent and spliced transcription units

Among 54 transcripts expressed in a temporal cascade during lytic infection with bovine herpesvirus 1, we have previously identified three major immediate-early (IE) RNAs, IER4.2 (4.2 kb), IER2.9 (2.9 kb), and IER1.7 (1.6 to 1.8 kb depending on the virus strain) transcribed from the HindIII C genome region (U. V. Wirth, K. Gunkel, M. Engels, and M. Schwyzer, J. Virol. 63:4882-4889, 1989). Northern (RNA) blot, S1 nuclease protection, and primer extension analysis used in the present study demonstrated that all three IE transcripts were spliced and originated from two divergent transcription units with start sites located in the inverted repeat. Transcription unit 1 encoded two alternative spliced transcripts, IER4.2 and IER2.9, with a common exon 1 located at 0.797 to 0.795 map units (m.u.) and an exon 2 for IER4.2 (0.792 to 0.762 m.u.) in the inverted repeat; exon 2 for IER2.9 (0.754 to 0.738 m.u.) was located in the unique long sequence and transcribed in antisense orientation to latency-related RNA. Transcription unit 2 (0.818 to 0.836 m.u.), further characterized by cDNA cloning, encoded the spliced IER1.7 with three exons in the inverted repeat. Additional minor IE transcripts were interpreted as unspliced precursors and splicing variants. With regard to the number and layout of IE genes, bovine herpesvirus 1 occupies an intermediate position between pseudorabies virus and equine herpesvirus 1 on the one hand and varicella-zoster virus and herpes simplex virus type 1 on the other.