Mapping of herpesvirus saimiri proteins on the viral genome: proteins dependent and not dependent on viral DNA synthesis

The herpesvirus saimiri ORF50 gene, encoding a transcriptional activator homologous to the Epstein-Barr virus R protein, is transcribed from two distinct promoters of different temporal phases

The mRNA species encoding the herpesvirus saimiri (HVS) homolog of the Epstein-Barr virus R transcriptional activator (termed ORF50) have been identified and used to determine transcriptional start sites within the gene. The first transcript is spliced and starts from a promoter within ORF49 containing a single intron; the second is produced from a promoter within the second exon and is in the same reading frame. The spliced transcript is detected at early times during productive virus replication in OMK cells, whereas the nonspliced transcript is detected later. The spliced transcript is fivefold-more potent in activating the delayed-early ORF6 promoter; the function of the nonspliced transcript is unclear. Thus, the role of this protein in activating herpesvirus saimiri from the latent state may differ significantly from that of the Epstein-Barr virus R protein.

Immediate-early, early, and late RNAs in bovine herpesvirus-4-infected cells

We have begun to identify immediate-early (IE), early (E), and late (L) genes of BHV-4 by analysis of cytoplasmic polyadenylated RNA transcribed from the BHV-4 genome over the course of infection and in the presence of cycloheximide or phosphonoacetic acid. Labeled cDNA prepared from RNA isolated at different times was hybridized with Southern blots of viral DNA to show which regions of the genome are transcribed at different times. In a second series of experiments, radiolabeled cloned restriction fragments representing the entire BHV-4 genome were hybridized separately to RNA on Northern blots to determine the number and sizes of transcripts at different times. As expected, with increasing time after infection, more portions of the BHV-4 genome are transcribed and a larger number and a greater abundance of viral transcripts are present. RNA transcribed from terminal repeats was not detectable at any time. However, similarity in size of RNA transcribed from opposite ends of the unique region of the genome late in infection suggests that RNA is transcribed over the fused ends of the genome, and terminal repeats are removed during RNA processing. Identification of IE, E, and L transcripts by this analysis lays the foundation for further study of specific BHV-4 transcripts and genes.

Analysis of nucleotide sequence of the rightmost 43 kbp of herpesvirus saimiri (HVS) L-DNA: general conservation of genetic organization between HVS and Epstein-Barr virus

We present an analysis of 43,658 bp of contiguous nucleotide sequence comprising the right terminal region (conventional orientation) of the unique protein-coding component (L-DNA) of the herpesvirus saimiri (HVS) genome. Within this region lie the genes encoding the 160-kDa virion protein, which is homologous to the 140-kDa membrane antigen of Epstein-Barr virus (EBV), thymidylate synthase (TS), and the immediate-early (IE) 52-kDa protein which is homologous to the EBV BMLF1 product. The 160-kDa gene of HVS lies at the right terminus of HVS L-DNA, its homologue in EBV occurring at the left terminus of the EBV genome (conventional orientation). The TS gene of HVS occurs within a group of 5 genes that have no homologues in EBV. The translation product of one of these genes, ECRF3, shows amino acid sequence and hydrophobicity pattern similarities to the HCMV and cellular G-protein-coupled receptor family of proteins. Another, ECLF2, is homologous to the cyclin family of cellular proteins. The 5 nonconserved genes lie adjacent to the 160-kDa gene. In EBV, the region to the right of the 140-kDa gene (BNRF1) contains the latent replication origin (OriP) and the open reading frames BCRF1, BWRF1 (repeated 12 times), BYRF1, BHLF1, and BHRF1, counterparts of which are not present in this position in HVS. The subsequent 18 genes in EBV (BFLF2 to BLRF2, approximate positions 56,000-89,500) are represented in HVS, and the relative positions and orientations of these genes are directly comparable between the two viruses. There then occurs a nonhomologous gene in HVS, and genes BLLF2 to BZLF1 (positions 89,500 to 103,200) in EBV which are not present in this region of HVS, before collinearity resumes. Thus, the HVS sequence presented here shows general collinearity between conserved genes in the right terminal region of HVS and the left terminal region of EBV and reveals the presence of two sets of unique genes which occur in exactly analogous positions in HVS and EBV.

The BI'LF4 trans-activator of Epstein-Barr virus is modulated by type and differentiation of the host cell

We have analyzed the activity and regulated expression of a new Epstein-Barr virus (EBV) trans-activator (I'ta) encoded by left reading frame 4 (BI'LF4) of the BamHI I'fragment. The gene was detected in all genomes of established EBV strains and individual isolates, with the exception of B95-8, where the type-specific deletion of this open reading frame is tolerated in vitro. Specific trans-activation of two EBV promoters (early MS and I'ta promoter) could be shown in cotransfection assays. The I'ta product affected autoactivation but had no influence on heterologous target promoters. The I'ta promoter segment was shown to be costimulated in the process of host cell differentiation in the absence of other EBV gene products. Expression of the reading frame in bacteria identified a 48-kDa protein as a stable gene product. I'ta-specific antibodies were detected in sera from EBV-positive persons (nasopharyngeal carcinoma). When expressed with suitable eucaryotic vectors, a nuclear protein could be immunostained in transfected cells. Our experiments suggest a cell type-specific requirement for I'ta in the lytic cycle of EBV at a determined differentiation stage of the host cell.

trans activation of the thymidylate synthase promoter of herpesvirus saimiri

Herpesvirus saimiri has been shown to possess a thymidylate synthase (TS) gene that is unusual in its transcriptional regulation. Although TS is believed to be required for viral DNA synthesis, the TS-specific 2.5-kb mRNA was found most abundantly during the late phases of asynchronous virus replication in permissive cultures. To study the kinetics of gene activation, the TS promoter and regulatory sequences were cloned upstream of the chloramphenicol acetyltransferase (CAT) gene. No CAT expression or transcripts were found after transfection of fusion genes into permissive owl monkey kidney (OMK) cells. However, the promoter was strongly activated when CAT plasmids were cotransfected with intact herpesvirus saimiri virion DNA or were transferred to OMK cells that were lytically infected with herpesvirus saimiri or a related herpesvirus, herpesvirus ateles. CAT was expressed at reduced levels in cultures when viral DNA replication was inhibited by phosphonoacetic acid; this indicates that the gene is activated during the delayed-early phase. However, the highest amounts of mRNA were present in the late period of replication. Deletion analyses localized essential response elements for trans activation in the promoter upstream region between nucleotides -311 and -56; they consisted of related tandem repeats and perfect palindromes. A sequence with two overlapping palindromes of 16 and 18 bp was found to be a major target for activation of the herpesvirus saimiri TS promoter. These palindromes did not have any significant homologies with known sequences of herpesviruses or cellular DNA; the 18-bp palindrome had, however, a certain structural similarity with a conserved sequence of the E2-responsive cis sequence that is required for transcription regulation of early papillomavirus genes.

Gene expression in cells infected with gammaherpesvirus saimiri: properties of transcripts from two immediate-early genes

During productive infections of cells with the gammaherpesvirus, herpesvirus saimiri (HVS), a polyadenylated RNA of 2.2-2.4 kb accumulates to form a large fraction of virus-specified RNA. This transcript is from the virus thymidylate synthase (TS) gene and its synthesis, like that of late mRNAs encoding the virus structural proteins, is sensitive to an inhibitor of virus DNA synthesis (phosphonoacetic acid, PAA). Transcription which is insensitive to PAA occurs from many parts of the HVS genome, including the EcoRI-D, EcoRI-E, EcoRI-I, and HindIII-G fragments. A 1.6-kb RNA from EcoRI-I/E and a 1.3-kb RNA from HindIII-G accumulate in HVS-infected cells incubated in the continuous presence of cycloheximide, and thus represent immediate-early (IE) class transcripts. The 1.3-kb message from HindIII-G is the predominant stable RNA under these conditions; accumulation of the 1.6-kb transcript from EcoRI-I/E (which encodes the previously characterized 52-kDa IE phosphoprotein) is markedly more dependent on the multiplicity of infection. The sequence of a 2.5-kbp region of the HindIII-G fragment has been determined and a single major open reading frame is present within the boundaries of the 1.3-kb IE RNA. Comparison of the amino acid sequence of the encoded protein (IE-G) with current databases of protein sequences failed to demonstrate significant similarities with herpesvirus proteins, but did detect a significant similarity with a region of the protein specified by an open reading frame in the LTR of mouse mammary tumor virus. The function of the IE gene in HindIII-G and the basis for the distinctive multiplicity dependence of IE transcription from the 52-kDa gene remain to be established.

Conservation of sequence and function between the product of the 52-kilodalton immediate-early gene of herpesvirus saimiri and the BMLF1-encoded transcriptional effector (EB2) of Epstein-Barr virus

We present a sequence of 2,220 nucleotides from a region of the genome of herpesvirus saimiri (HVS) which includes the coding and putative regulatory sequences for the 52-kilodalton (kDa) immediate-early (IE) phosphoprotein of the virus. The amino acid sequence predicted for this protein shows it to be homologous to the EB2 transcriptional effector encoded by the BMLF1 open reading frame of Epstein-Barr virus (EBV), the IE 68-kDa protein of varicella-zoster virus, and the IE 63-kDa (alpha 27) protein of herpes simplex virus (HSV). By measuring the function of the HVS 52-kDa-protein gene in transient expression assays, we also showed that it can substitute with comparable efficiency for the EB2 product of EBV in the EB1-dependent activation of the EBV DR promoter. The alpha 27 gene of HSV was an inefficient trans-activator in similar assays. We conclude that the IE 52-kDa protein of HVS is structurally and functionally more similar to the homologous protein of the human lymphotropic virus, EBV, than to the corresponding proteins from the neurotropic viruses, varicella-zoster virus and HSV.

Conservation of gene organization in the lymphotropic herpesviruses herpesvirus Saimiri and Epstein-Barr virus

By analyses of short DNA sequences, we have deduced the overall arrangement of genes in the (A + T)-rich coding sequences of herpesvirus saimiri (HVS) relative to the arrangements of homologous genes in the (G + C)-rich coding sequences of the Epstein-Barr virus (EBV) genome and the (A + T)-rich sequences of the varicella-zoster virus (VZV) genome. Fragments of HVS DNA from 13 separate sites within the 111 kilobase pairs of the light DNA coding sequences of the genome were subcloned into M13 vectors, and sequences of up to 350 bases were determined from each of these sites. Amino acid sequences predicted for fragments of open reading frames defined by these sequences were compared with a library of the protein sequences of major open reading frames predicted from the complete DNA sequences of VZV and EBV. Of the 13 short amino acid sequences obtained from HVS, only 3 were recognizably homologous to proteins encoded by VZV, but all 13 HVS sequences were unambiguously homologous to gene products encoded by EBV. The HVS reading frames identified by this method included homologs of the major capsid polypeptides, glycoprotein H, the major nonstructural DNA-binding protein, thymidine kinase, and the homolog of the regulatory gene product of the BMLF1 reading frame of EBV. Locally as well as globally, the order and relative orientation of these genes resembled that of their homologs on the EBV genome. Despite the major differences in their nucleotide compositions and in the nature and arrangements of reiterated DNA sequences, the genomes of the lymphotropic herpesviruses HVS and EBV encode closely related proteins, and they share a common organization of these coding sequences which differs from that of the neurotropic herpesviruses, VZV and herpes simplex virus.

The 160,000-Mr virion protein encoded at the right end of the herpesvirus saimiri genome is homologous to the 140,000-Mr membrane antigen encoded at the left end of the Epstein-Barr virus genome

The sequence of 4.4 kilobase pairs (kbp) from the conventional right terminus of the A + T-rich light-DNA (L-DNA) sequences of the herpesvirus saimiri (HVS) genome contains a leftward-directed open reading frame (ORF) for a 1,299-residue protein. The molecular weight predicted for the protein (143,000) is in good agreement with the estimates of 150,000 to 160,000 for the major nonglycosylated polypeptide of the virion tegument (the 160K polypeptide), previously shown to be encoded by this region of the genome. The first initiation codon of the ORF is only 250 nucleotides from the junction of the L-DNA component with the G + C-rich terminal reiterations (i.e., heavy or H-DNA) of the genome. An unusually A + T-rich sequence (43 of 45 nucleotides are A or T, relative to a mean composition of 40% G + C for the ORF) occurs some 75 bp 5' to this initiation codon, and the first adenylation signal (AATAAA) on this DNA strand occurs 18 bp 3' to the termination codon. The amino acid sequence predicted for the 160K protein of HVS is homologous over most of its length to the 1,318-residue protein encoded by the leftmost major ORF of the G + C-rich genome of Epstein-Barr virus (BNRF1, the 140K nonglycosylated membrane antigen). No homology to either of these proteins is evident among the products predicted from the complete sequence of the alpha herpesvirus varicella-zoster virus. Thus gamma herpesviruses with coding sequences which differ in mean nucleotide composition by some 20% G + C have homologous proteins encoded at similar positions with respect to genome termini, with the right end of HVS being homologous to the left end of Epstein-Barr virus.