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Journal of Virology, March 2004, p. 2609-2614, Vol. 78, No. 5
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.5.2609-2614.2004
Requirement of a 12-Base-Pair TATT-Containing Sequence and Viral Lytic DNA Replication in Activation of the Kaposi's Sarcoma-Associated Herpesvirus K8.1 Late Promoter
Shuang Tang, Koji Yamanegi, and Zhi-Ming Zheng*
HIV and AIDS Malignancy Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892
Received 28 May 2003/
Accepted 25 November 2003

ABSTRACT
Kaposi's sarcoma-associated herpesvirus (KSHV) K8.1 late promoter
consists of a minimal 24-bp sequence, with a TATA-like, 12-bp
promoter core, AATATTAAAGGG, and is active on a reporter only
in butyrate-induced KSHV-infected cells. The activity of the
K8.1 promoter can be enhanced (>15-fold) by the KSHV left-end
lytic origin of DNA replication (oriLyt-L) sequence while providing
inefficient replication of plasmid DNA and is inhibited by viral
DNA replication inhibitors, suggesting that activation of the
K8.1 promoter on the reporter is involved in KSHV lytic DNA
replication largely by
trans.

INTRODUCTION
Kaposi's sarcoma-associated herpesvirus (KSHV) closely resembles
human Epstein-Barr virus (EBV) (
4). Like other gammaherpesviruses,
KSHV infection displays two viral life cycles. Latent KSHV infection
in KS tissues and B-cell lines features the highly restricted
expression of only five genes (
7,
33). The lytic KSHV life cycle
is characterized by production of progeny virus from infected
cells and can be induced by tetradecanoyl phorbol acetate and
n-butyrate in PEL-derived B cells with latent KSHV infection
(
21,
22,
24).
KSHV K8.1 is a viral late gene that encodes a viral envelope glycoprotein at a late stage of virus infection. The sensitivity of K8.1 expression to a viral DNA polymerase inhibitor, phosphonoacetic acid (PAA) (17, 29), suggests that K8.1 is a true late gene (
2) whose expression, like that of other herpesvirus
2 genes (11, 19, 28, 30, 31), depends on viral DNA replication.
Transcription of the K8.1 late gene in butyrate-induced KSHV-infected JSC-1 cells starts at nucleotide (nt) 75901, 14 nt upstream of the first AUG codon at nt 75915 in the virus genome (29). As a result, a putative K8.1 promoter was hypothesized to exist upstream of the start site for transcription initiation.
To study the putative K8.1 promoter, we had to overcome some technical limitations. The putative predicted K8.1 promoter overlaps two KSHV genes: ORF50 and K8 (32). ORF50 encodes a transactivator, RTA (replication and transcription activator), responsible for initiating all KSHV early gene expression. K8 is a gene encoding a leucine zipper protein involved in viral DNA replication (16). Mutation in this region in the context of the virus genome will affect expression of the ORF50 and K8 genes and viral DNA replication. Thus, it is not feasible to study the K8.1 late promoter in its native context. As an alternative, we established a transient-transfection assay to study the putative K8.1 promoter, using various sizes of PCR-amplified fragments immediately upstream of nt 75915, but not overlapping the K8 promoter region (17). All fragments were cloned into the KpnI and XhoI sites upstream of the firefly luciferase open reading frame (ORF) in a pGL3 luciferase reporter vector lacking eukaryotic origins of replication (Promega, Madison, Wis.). The activity of each promoter-firefly luciferase reporter construct was examined from cell lysate supernatant by a dual luciferase assay (Promega) 48 h after transfection of 3 µg of the construct, together with 2 µg of Renilla luciferase reporter (RL-TS) (13) and 14 µg of sheared salmon sperm DNA into B lymphocytes (5 x 106 cells) by electroporation at 250 V and 950 µF with an Electro Cell Manipulator (BTX ECM630; BTX, San Diego, Calif.).

KSHV K8.1 promoter activity is induced in KSHV-infected cells only by trans.
Plasmid pST7 (Fig.
1C) was first examined for its promoter activity
in 293 cells and showed no activity with or without chemical
stimulation and ORF50 and/or K8 cotransfection (data not shown).
This experiment excluded the possibility of an immediate-early
or early promoter-like activity residing in this construct.
Next, we examined the promoter activity of the pST7 plasmid
in several B-cell lines, including Raji (EBV-infected but replication-defective)
cells (
6), SUDHL6 cells (a B-cell line with no virus infection)
(
18), JSC-1 (KSHV-infected, EBV-infected cells), and BCBL-1
(KSHV-infected, non-EBV-infected) cells. The pST7 plasmid conferred
no or very little promoter activity in either butyrate-stimulated
or unstimulated Raji cells and SUDHL6 cells but conferred strong
promoter activity in both butyrate-stimulated JSC-1 (Fig.
1A)
and BCBL-1 cells (Fig.
1B) that was comparable to that of the
simian virus 40 (SV40) early promoter (pGL3-SV40 vector in Fig.
1A and B), suggesting that a specific KSHV, but not EBV, protein(s)
must be involved in
trans activation. This regulation has to
do with viral lytic but not latent protein(s), since uninduced
JSC-1 and BCBL-1 cells did not support much K8.1 promoter activity.

Mapping of the K8.1 promoter to a functional promoter core conferring full promoter activity in JSC-1 cells.
To identify the transcription factor-binding sites that might
regulate the activity of the K8.1 promoter in viral lytic induction,
a series of successive deletions in the 1-kb K8.1 promoter was
made from its 5' end progressively to its 3' end (Fig.
1C).
The deletion mutants did not show much difference in their promoter
activity in butyrate-stimulated JSC-1 cells. A 58-bp region
proximal to the 3' end (pST18) contributed a similar amount
of promoter activity, as did the 1-kb K8.1 promoter (pST7) (Fig.
1C), indicating that the 58-bp region fully functions as a K8.1
promoter. In addition, a series of plasmids having various 3'
extensions of the 1-kb K8.1 promoter to include additional sequences
up to 73 bp downstream of nt 75915 showed only a minimal effect
on K8.1 promoter activity (data not shown). Together, these
results demonstrate that a functional K8.1 promoter is not subject
to regulation by a
cis-acting transcription factor-binding site
either upstream or downstream of the 58-bp K8.1 promoter.
To better define which sequences within the 58-bp region specify K8.1 promoter activity, another series of deletion or point mutations were made in the 58-bp promoter. Examination of the promoter activity of each mutant in butyrate-stimulated JSC-1 cells (Fig. 2A) demonstrated that the K8.1 promoter activity resides in the 24 bp at its 5' half (pST41) and is orientation sensitive (pST36), suggesting its independence on an initiator (Inr) at the transcription start site or downstream activation sequences (DAS) as described in other viral late promoters (8, 10, 14, 15, 23). The sequence of the 5' half of the 58-bp promoter, CCGGCAGCAATATTAAAGGGACCC, features a central TATA-like motif, TATT, flanked by several A's. To characterize the function of this putative TATA-like motif, we chose a 3'-truncated 45-bp promoter (pST34) with promoter activity similar to that of the 58-bp promoter (pST18) in butyrate-induced JSC-1 cells for introduction of point mutations. The conversion of TT to CG in the TATT motif (pST37) effectively disrupted its activation by butyrate in JSC-1 cells.
Having identified that a 24-bp sequence in the 5' half of the
58-bp K8.1 promoter is responsible for the promoter activity
and that the TATT motif within the 5' half functions as a TATA
box, we wished to determine whether the sequences flanking this
TATT box are important for the sufficient and active function
of the TATA-like box. A randomized 3-bp linker-scanning mutation
was introduced progressively from 5' to 3' into the 24-bp K8.1
promoter, and each mutant was analyzed in butyrate-stimulated
JSC-1 cells. As shown in Fig.
2B, the sequences flanking the
TATT box were demonstrated to play important roles in
trans activation of the 24-bp promoter by butyrate. Interestingly,
the wild-type (wt) but not mutant 24-bp promoter by itself was
capable of binding the TATA-binding protein (TBP) in our gel
shift assays (Fig.
2C). Thus, a K8.1 late promoter core was
identified; it consists of a 12-bp sequence (AATATTAAAGGG),
including the TATT box at position -34 relative to the K8.1
transcription start site and its flanking sequences immediately
upstream and downstream.
The size of the crucial 12-bp K8.1 promoter core is similar to that of the herpes simplex virus type 1 gC late promoter core (12). Both are embedded in GC-rich flanking sequences and contain signals essential for fully regulated activation. However, the gC promoter core has a sequence, GGGTATAAATTCCGG, which deviates from the consensus Goldberg-Hogness sequence (GGGTATAAATA) (2) by only 1 nt, whereas the defined K8.1 12-bp promoter core, like the SV40 11-bp late promoter core (GGTACCTAACC) (2), deviate much further from the Goldberg-Hogness sequence. A TATT box, deviating by 1 nt at the fourth position from the sequence of the classical TATA box for TBP binding to recruit RNA polymerase II, appears to be common in many viral late promoters, including cytomegalovirus UL94 (31) and UL75 (20), EBV BcLF1 (26), and KSHV AP (3).

Sensitivity of trans-activated K8.1 late promoter activity to PAA and GCV in butyrate-induced JSC-1 cells.
To examine whether the K8.1 promoter identified in our transient-transfection
assay could represent an authentic late promoter in responding
to viral DNA replication inhibitors, two versions of the K8.1
promoter, a 1-kb promoter in pST7 and a 24-bp promoter in pST41,
were further analyzed in butyrate-induced JSC-1 cells in the
presence or absence of PAA or ganciclovir (GCV). The activities
of both promoters were sensitive to PAA, but not GCV (Fig.
3A and B),
with PAA inhibiting promoter activity by up to 60%,
indicating an association with viral lytic DNA replication.
Although PAA did not inhibit the transient K8.1 promoter activity
completely, this was expected, since neither of the vectors
containing the K8.1 promoter was able to replicate, due to lack
of eukaryotic origins of DNA replication. This was further supported
by data showing that the transient promoter activity in butyrate-induced
JSC-1 cells was insensitive to a DNA chain elongation terminator,
GCV (
5). However, transcription of the late gene K8.1 in JSC-1
cells was sensitive to both PAA (96% inhibition) and GCV (87%
inhibition) (Fig.
3C), suggesting that our transient promoter
assay reveals only a partial recapitulation of activation of
the authentic late promoter in the context of the virus genome.

A viral oriLyt functions in cis, acting to enhance activation by trans of the K8.1 late promoter.
To overcome the replication incompetence of the plasmids described
above, a KSHV left-end lytic origin of DNA replication (oriLyt-L)
(
1) was inserted downstream of the luciferase ORF driven by
the K8.1 promoter. Five original K8.1 promoter plasmids (pST41,
pST7, pST26, pST36, and pST46) were chosen for insertion of
the KSHV oriLyt-L sequence (Fig.
4A), and their promoter activities
were examined in butyrate-induced JSC-1 cells. As shown in Fig.
4B, once the oriLyt sequence was inserted, the promoter activities
of plasmids with a wt K8.1 promoter either in a long (1-kb)
or short (24-bp) version (pST52, pST53, pST60, pST54, and pST55)
were greatly enhanced (more than 15-fold) with butyrate, independent
of oriLyt orientation. The same sets of plasmids with a wt K8.1
promoter plus the viral oriLyt were also tested in butyrate-induced
BCBL-1 cells and 293 cells. The plasmids reproducibly showed
the same results only in butyrate-induced BCBL-1 cells, not
in 293 cells (data not shown). In contrast, the K8.1 promoter
with mutations in its TATT box or in an antisense orientation
did not have enhanced activity in the presence of oriLyt (pST61,
pST57, pST58, and pST59 in Fig.
4B). This was expected, since
the original plasmids (pST46 and pST36) lacking the viral oriLyt
sequence had no promoter activity (Fig.
2A and B).
The much higher level of K8.1 promoter activity in butyrate-induced
JSC-1 cells with the viral oriLyt-containing plasmid p53 (Fig.
4A) was sensitive to PAA and GCV, with an inhibition up to 97%
by PAA and 80% by GCV (Fig.
4C). Thus, the 24-bp promoter on
this plasmid appeared to fully imitate the activation of an
authentic K8.1 promoter in responding to viral lytic DNA replication
and DNA replication inhibitors in the context of the virus genome.
The finding that GCV, a DNA chain elongation terminator, inhibited
the oriLyt-enhanced K8.1 promoter activity provides further
evidence that KSHV lytic DNA replication, including DNA chain
elongation, is itself directly engaged in activation of the
late promoter on the reporter.
To determine whether the viral oriLyt-enhanced K8.1 promoter activity is attributable to plasmid replication, a transient-replication assay (1) was conducted in parallel with pST53. As shown in Fig. 4D, inserting KSHV oriLyt in pST53 conferred only a very low level of plasmid DNA replication in butyrate-induced JSC-1 cells, as evidenced by the presence of some DpnI-resistant DNA (lane 6) and its sensitivity to MboI digestion (lane 7) and to DNA replication inhibitors PAA and GCV (lanes 8 and 9). These results indicate that maintenance of plasmid DNA replication or template (copy number) amplification is unlikely to play a major role in inducing a much higher level of activation of the K8.1 promoter on the reporter, as presented in a study on the cytomegalovirus late 1.2-kb RNA promoter (30).
The specific trans-acting factors important to K8.1 promoter activation in KSHV-infected cells appear to be independent of viral ORF50, since cotransfection with ORF50 did not promote much promoter activity of both replication-competent and -incompetent plasmids (data not shown). If ORF50 has any effect, it might be indirect. We speculate that this trans-acting factor(s) may be a component(s)-perhaps a viral early gene product(s)-of the viral lytic, but not latent, DNA replication (initiation and elongation) machinery, since uninduced JSC-1 cells did not support much K8.1 promoter activity. A similar observation in the EBV late promoters BcLF1 and BFRF3 supports such a trans relationship (27). Although the underlying mechanisms that link the late promoter activation and KSHV lytic DNA replication remain unknown and the reporter assays in this study may not fully reflect regulation in the intact virus, the finding that KSHV K8.1 late promoter can be activated largely by trans in viral lytic DNA replication provides a new insight into regulation of KSHV late gene expression.

ACKNOWLEDGMENTS
We thank Richard Ambinder of the Johns Hopkins University School
of Medicine for providing JSC-1 cells, Louis Staudt of NCI for
providing SUDHL6 cells, Hui Ge of ProteinOne for providing yeast
TBP protein, and David AuCoin and Gregory Pari of the University
of Nevada School of Medicine for providing plasmid pDA13 for
this study. BCBL-1 cells were obtained from Michael McGrath
and Don Ganem through the AIDS Research and Reference Reagent
Program, Division of AIDS, NIAID, NIH. We also thank John Brady
and Robert Yarchoan for critically reading the manuscript.

FOOTNOTES
* Corresponding author. Mailing address: HIV and AIDS Malignancy Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bldg. 10, Rm. 10S255, MSC 1868, 10 Center Dr., Bethesda, MD 20892-1868. Phone: (301) 594-1382. Fax: (301) 480-8250. E-mail:
zhengt{at}exchange.nih.gov.


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Journal of Virology, March 2004, p. 2609-2614, Vol. 78, No. 5
0022-538X/04/$08.00+0 DOI: 10.1128/JVI.78.5.2609-2614.2004
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