Coding content and expression of the EBV B95-8 genome in the region from base 62,248 to base 82,920

Marked variation in response of consensus binding elements for the Rta protein of Epstein-Barr virus

The R transactivator (Rta) protein activates Epstein-Barr virus (EBV) lytic-cycle genes by several distinct mechanisms that include direct binding to viral promoters, synergy with BamHI Z EBV replication activator (ZEBRA), and activation of cellular signaling pathways. In the direct and synergistic mechanisms of action, Rta binds to specific DNA sequences that are present in the promoters of responsive genes. It has been difficult to demonstrate the capacity of Rta expressed in mammalian cells to bind DNA in vitro in order to study the relative affinities of Rta binding elements. We discovered that a short C-terminal region of Rta inhibits the ability of Rta to bind DNA in vitro. C-terminally truncated versions of Rta bind DNA efficiently and thus facilitate a comparison of consensus Rta binding elements (CRBEs) found in promoters of five Rta-responsive genes: BMLF1, BHLF1, BMRF1, BaRF1, and BLRF2. All CRBEs in the promoters of the five genes conform to the proposed recognition sequence GNCCN9GGNG, where N is any nucleotide and N9 represents a sequence of nine nucleotides. Nonetheless, CRBEs varied markedly in their abilities to bind Rta in electrophoretic mobility shift assays. Not all CRBEs bound or responded to Rta. Binding affinities of the CRBEs and the capacity to be activated by Rta in reporter assays were strongly correlated. The CRBEs from the BMLF1 and BHLF1 promoters conferred the greatest response. The response of the BMRF1, BaRF1, and BLRF2 CRBEs was less robust. By creation of chimeras, inversions, and point mutations, differences in binding affinities and transcriptional activation levels could be attributed to N9 sequence variation. The length of N9 was also critical for a maximal response. In Raji and BZLF1-knockout cells, the mRNAs of the five Rta-responsive lytic-cycle genes differed dramatically in kinetics of expression, abundance, and synergistic responses to ZEBRA and Rta. Affinities of Rta response elements for Rta are likely to play an important role in temporal regulation and the level of lytic-cycle EBV gene expression.

Role of the epstein-barr virus RTA protein in activation of distinct classes of viral lytic cycle genes

Initiation of the Epstein-Barr virus (EBV) lytic cycle is controlled by two immediate-early genes, BZLF1 and BRLF1. In certain epithelial and B-cell lines, their protein products, ZEBRA and Rta, stimulate their own expression, reciprocally stimulate each other's expression, and activate downstream viral targets. It has been difficult to examine the individual roles of these two transactivators in EBV-infected lymphocytes, as they are expressed simultaneously upon induction of the lytic cycle. Here we show that the Burkitt lymphoma cell line Raji represents an experimental system that allows the study of Rta's role in the lytic cycle of EBV in the absence and presence of ZEBRA. When expressed in Raji cells, exogenous Rta does not activate endogenous BZLF1 expression, yet Rta remains competent to transactivate certain downstream viral targets. Some genes, such as BaRF1, BMLF1, and a late gene, BLRF2, are maximally activated by Rta itself in the absence of detectable ZEBRA. The use of the Z(S186A) mutant form of ZEBRA, whose transactivation function is manifest only by coexpression of Rta, allows identification of a second class of lytic cycle genes, such as BMRF1 and BHRF1, that are activated in synergy by Rta and ZEBRA. It has already been documented that of the two activators, only ZEBRA stimulates the BRLF1 gene in Raji cells. Thus, there is a third class of viral genes activated by ZEBRA but not Rta. Moreover, ZEBRA exhibits an inhibitory effect on Rta's capacity to stimulate the late gene, BLRF2. Consequently ZEBRA may function to repress Rta's potential to activate some late genes. Raji cells thus allow delineation of the combinatorial roles of Rta and ZEBRA in control of several distinct classes of lytic cycle genes.

Physical mapping and nucleotide sequence of a herpes simplex virus type 1 gene required for capsid assembly

In this report, we describe some phenotypic properties of a temperature-sensitive mutant of herpes simplex type 1 (HSV-1) and present data concerning the physical location and nucleotide sequence of the genomic region harboring the mutation. The effect of shifts from the permissive to the nonpermissive temperature on infectious virus production by the mutant A44ts2 indicated that the mutated function is necessary throughout, or late in, the growth cycle. At the nonpermissive temperature, no major differences were detected in viral DNA or protein synthesis with respect to the parent A44ts+. On the other hand, electron microscopy of mutant-infected cells revealed that neither viral capsids nor capsid-related structures were assembled at the nonpermissive temperature. Additional analyses employing the Hirt extraction procedure showed that A44ts2 is also unable to mature replicated viral DNA into unit-length molecules under nonpermissive conditions. The results of marker rescue experiments with intact A44ts2 DNA and cloned restriction fragments of A44ts+ placed the lesion in the coordinate interval 0.553 to 0.565 (1,837 base pairs in region UL) of the HSV-1 physical map. No function has previously been assigned to this region, although it is known to be transcribed into two 5' coterminal mRNAs which code in vitro for a 54,000-molecular-weight polypeptide (K. P. Anderson, R. J. Frink, G. B. Devi, B. H. Gaylord, R. H. Costa, and E. K. Wagner, J. Virol. 37:1011-1027, 1981). We sequenced the interval 0.551 to 0.565 and found an open reading frame (ORF) for a 50,175-molecular-weight polypeptide. The predicted product of this ORF exhibits strong homology with the product of varicella-zoster virus ORF20 and lower, but significant, homology with the product of Epstein-Barr virus BORF1. For the three viruses, the corresponding ORFs lie just upstream of the gene coding for the large subunit of viral ribonucleotide reductase. The ORF described here corresponds to the ORF designated UL38 in the recently published nucleotide sequence of the HSV-1 UL region (D. J. McGeoch, M. A. Dalrymple, A. J. Davison, A. Dolan, M. C. Frame, D. McNab, L. J. Perry, J. E. Scott, and P. Taylor, J. Gen. Virol. 69:1531-1574, 1988).

Altered expression of two Epstein-Barr virus early genes localized in BamHI-A in nonproducer Raji cells

The Epstein-Barr virus-carrying lymphoblastoid cell line Raji has two major genomic deletions and is incapable of virus production. Two cDNA clones, c70 and c55, were constructed from early mRNA of P3HR-1 cells and localized, respectively, in BALF-2 and BARF-1 open reading frames where one of the major genomic deletion in Raji cells is situated. These were used to search the different early viral transcripts in producer P3HR-1 and nonproducer Raji lines. c70 and c55 hybridized with their corresponding mRNAs only in producer lines. Analysis with in vitro-synthesized RNA probes showed quite a different transcriptional profile in Raji cells than in P3HR-1 cells. In the P3HR-1 line, BALF-2 encodes a 3.4-kilobase (kb) mRNA during the early phase and a 3.3-kb mRNA during the late phase, and in the Raji line, the probe corresponding to BALF-2 hybridized with three mRNAs of 5.0, 3.1, and 2.4 kb; in P3HR-1 cells, BARF-1 encodes a group of 3'-conterminal transcripts (0.8, 1.2, 1.7, 2.7, 3.2, and 5.0 kb) during both the early and late stages; in Raji cells, however, 0.8-, 1.2-, and 1.7-kb mRNAs are absent, the only mRNAs transcribed being upstream of the deletion and of 5.0, 2.6, and 2.0 kb in size. In vivo and in vitro experiments demonstrated that the BALF-2 open reading frame encodes an early 135-kilodalton (kDa) protein which possesses DNA-binding ability and can be recognized by a herpes simplex virus ICP-8 antiserum. The BARF-1 open reading frame encodes in vitro a 26- to 33-kDa early protein recognized by anti-EA serum. The proteins of both two genes expressed in psi AM 22b cells were localized in nuclei. According to their properties, both proteins, particularly the BALF-2-encoded 135-kDa DNA-binding protein, could play a role in virus replication.

A restricted component of the Epstein-Barr virus early antigen complex is structurally related to ribonucleotide reductase

The 85-kDa polypeptide previously shown to be associated with the restricted (R) component of the EBV-induced early antigen (EA) complex was subjected to amino acid sequencing analysis. This was accomplished by cyanogen bromide cleavage and by separation of individual peptides by high-pressure liquid chromatography employing reversed-phase C18 column techniques. Two of the isolated peptides, F11 and F13, were subjected to amino acid sequencing and both were found to have significant homology to the postulated protein encoded by the BamHI O right reading frame 2 (BamHI ORF2) of the B95-8 strain Epstein-Barr virus. Computer analysis revealed significant homology between the amino acid sequence of this polypeptide and ribonucleotide reductase, an enzyme previously mapped to this genomic fragment. Amino acid composition analysis also revealed a similar association. These results indicate that the 85-kDa EA(R) polypeptide is associated with a component of the EBV-induced ribonucleotide reductase.

Deletion mutants in the gene encoding the herpes simplex virus type 1 immediate-early protein ICP0 exhibit impaired growth in cell culture

We report the construction and characterization of deletion mutants in the herpes simplex virus type 1 gene encoding the immediate-early protein ICP0. In the event that ICP0 proved to play an essential role in virus replication, ICP0-transformed Vero cells were generated to serve as permissive hosts for such mutants. Two mutants, dlX0.7 and dlX3.1, were isolated in these cells by a marker rescue-transfer procedure involving the rescue of an ICP4 deletion mutant and the simultaneous insertion of a linked deletion in the ICP0 gene. Mutant dlX0.7 contained a 700-base-pair deletion in both copies of ICP0. The deletion lay entirely within the transcript specified by the gene. dlX0.7 induced the synthesis of an ICP0-specific mRNA that was approximately 0.7 kilobases smaller than the corresponding mRNA specified by wild-type virus. The 3.1-kilobase deletion in both copies of the ICP0 gene in mutant dlX3.1 removed the majority of the transcriptional-regulatory signals and coding sequences, retaining only sequences at the 3' end of the gene. As expected, no ICP0-specific mRNA was detected in dlX3.1-infected Nero cells (G418-resistant Vero cells). Both mutants grew in all cells tested, although their burst sizes were 10- to 100-fold lower than that of wild-type virus. Although the plaque sizes of dlX0.7 and dlX3.1 were equally small on Nero and ICP0-transformed cells, the plating efficiency of the mutants was 15- to 50-fold greater on ICP0-transformed cells than on Nero cells. The mutants exhibited modest interference with the growth of wild-type virus in mixed infections, an effect that was abolished by UV irradiation of the mutants, implying that interference required viral gene expression. Polypeptide profiles generated by the mutants in Nero cells were qualitatively similar to that of wild-type virus. Quantitatively, only slight reductions in the levels of certain late viral polypeptides were observed, a phenomenon also borne out by analysis of viral glycoproteins. Both mutants induced the synthesis of significant, although reduced, levels of viral DNA relative to wild-type virus. Taken together, the results demonstrate that ICP0 is not essential for productive infection in cell culture but that this protein plays a significant role in viral growth, as indicated by the impaired abilities of the mutants to replicate.