Expression of class I and class II major histocompatibility antigens on human hepatocellular carcinoma.

Previous reports indicate that human hepatocytes do not express class I and class II MHC antigens. Our analyses on 10 human hepatocellular carcinoma (HCC) cell lines by immunofluorescence tests and RIA, demonstrate that all the human HCC cell lines tested express class I MHC antigens and among them, three poorly differentiated human HCC cell lines also express class II MHC antigens. Results of immunoprecipitation and/or Western blotting experiments indicate similarity in the chemical nature of both the class I and class II MHC antigens expressed by the human HCC cell lines and by a human B lymphoblastoid cell line Raji. Furthermore, a new variant form of class I antigen was detected in some of these HCC cell lines. Immunohistochemical studies of HCC tissues using the peroxidase-antiperoxidase staining method indicated that class I and class II antigens were detectable in 7 out of 11 and 3 out of 11 HCC tissues from patients, respectively. The availability of MHC class I antigen-positive cultured HCC cell lines, including the poorly differentiated lines that also express MHC class II antigen, has provided us with interesting models to study the relationship between expression of MHC antigen and transformation and differentiation of human hepatocytes. These studies will also allow us some insight into the role of MHC class I and class II antigen in the immunosensitivity and immunogenicity of HCC cells to the host-immune response.

Selective suppression of insulin-induced proliferation of cultured human hepatoma cells by somatostatin.

The effects of somatostatin (SRIF), insulin, and triiodothyronine (T3) on the growth of human hepatoma cells were investigated on the well-differentiated human hepatoma cell line Hep3B. Results showed that both insulin and T3 can stimulate cell growth of serum starved Hep3B cells at physiological concentrations. SRIF alone showed little growth-promoting activity. When added concurrently with insulin, however, SRIF suppressed the insulin-induced cell proliferation in a dose-dependent manner. On the other hand, SRIF had no inhibitory effect on T3-induced cell proliferation. SRIF is labile in the medium, with a half-life of about 2 h during culture incubation. SRIF did not disturb the insulin binding to its surface receptors nor inhibit the insulin-dependent receptor kinase activity of Hep3B cells in vitro. These results suggest that postreceptor regulation may be involved. The selective suppression by SRIF of insulin-induced cell growth provides an unique approach to the study of insulin actions on proliferation of human hepatoma cells.

C/EBP-like proteins binding to the functional box-alpha and box-beta of the second enhancer of hepatitis B virus.

The second enhancer (enhancer II) of hepatitis B virus is functionally liver specific. Located within an open reading frame of the virus and immediately upstream of the initiation sites of viral major transcripts, enhancer II furnishes a unique model for use in investigating the structure and function of an enhancer. In this study, two functional constituents, a 23-bp box-alpha and a 12-bp box-beta, are identified as being both necessary and sufficient for enhancer II function. Examination of the box-alpha and box-beta sequences reveals a weak homology to the extended consensus for a C/EBP binding site. Gel shift and footprinting analyses indicate that multiple proteins bind to these sequences and thus are candidate transcription factors that mediate the enhancer function. One heat-resistant protein, protein a, and one heat-sensitive protein, protein b, bind to box-alpha. Protein a, which binds to box-alpha in a way indistinguishable from that seen with a recombinant C/EBP, appears not to be identical to C/EBP in that the binding of protein a requires a minimal sequence larger than the canonical C/EBP sites. Two box-beta-binding proteins, c and d, show greater affinity for the C/EBP consensus than for box-beta. However, both proteins c and d are relatively heat sensitive and display a distinct sequence preference from the recombinant C/EBP protein. Since the function of enhancer II is strictly dependent on a bipartite architecture, this system provides a unique model for studies of how the interactions of its binding proteins lead to the enhancer function.

A liver-specific nuclear factor interacts with the promoter region of the large surface protein gene of human hepatitis B virus.

The outer envelope of the 42-nm virion of the human hepatitis B virus (HBV) is composed of the large, the middle, and the major surface proteins. Whereas the middle and the major surface proteins are transcribed from the SPII promoter of the pre-S/S gene, the large surface protein is transcribed from the SPI promoter located upstream of SPII. We have previously shown that transcription of SPI (comprising nucleotides [nt] -380 to +17) occurs preferentially in differentiated hepatoma cell lines (H.K. Chang and L.P. Ting, Virology 170:176-183, 1989). In this report, we further demonstrated that a sequence of 95 base pairs in the upstream region of SPI (nt -95 to +17) was necessary and sufficient for such preferential expression in differentiated hepatoma cells. By analysis of the expression of the chloramphenicol acetyltransferase gene in a series of mutants with deletions at the 5' end of SPI, we identified a positive transcriptional cis-acting element mapping at nt -95 to -72 which appears to play a key role in the regulation of the expression of the large surface protein. This region shared a high degree of sequence homology with regulatory sequences of several liver-specific genes from human, mouse, and rat, with a consensus sequence (G/A)GTTA(A/C)TNNT(C/T)NNC(A/C). We further identified a nuclear factor present in the nuclear extracts of differentiated human hepatoma cell lines which interacted specifically with this element of the SPI promoter. This nuclear factor was similar to the rat liver-specific factor HNF-1, since an oligonucleotide containing the recognition sequence of HNF-1 could efficiently compete for the human factor in a footprinting assay. The sequence at nt -93 to -68 which was bound by this factor in SPI was termed the HNF-1-binding element. Activation of the SPI promoter by human differentiated hepatocyte nuclear factor 1, described in this report, probably explains, first, the formation of the 42-nm virion specifically in liver but not in several other tissues despite the synthesis of the middle and the major surface proteins in those tissues, and second, why only differentiated hepatoma cell lines are able to produce 42-nm-like virion particles on transfection by HBV DNA.

Efficiency of proton extrusion by chemically modified mitochondria.

Bovine heart mitochondria were treated with limited amounts of iodoacetamide, 1-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline, phenylglyoxal, tetranitromethane and 1-fluoro-2,4-dinitrobenzene, respectively. Examination of the respiration and proton extrusion characteristics of the chemically modified mitochondria suggests that sulfhydryl and imidazole groups are not directly involved in proton pumping, but that some of the labeled carboxyl, amino, guanidinium and phenolic groups may participate in an indirect proton-extrusion process. Cross-linking mitochondria with glutaraldehyde drastically decreases the efficiency of proton extrusion, whereas treatment of mitochondria with valeraldehyde under similar conditions did not affect the proton-pumping efficiency significantly. The latter observations show that conformational change in the inner mitochondrial membrane may play a crucial role in the active translocation of protons coupled to electron transport. Comparison of the reactivities of the essential amino and carboxyl groups in mitochondria in different oxidation states suggests that these two types of essential functional groups are more exposed to water in the oxidized state. An indirect mechanism for proton pumping based on protein conformational change driven by electron transport based on the results of the present chemical modification studies is suggested.

Differential regulation of major surface promoter in hepatitis B virus.

The major surface promoter of human hepatitis B virus can produce three distinct groups of S transcripts. The initiation sites of these transcripts are in close proximity. Encompassing the ATG for the middle surface protein, the largest S transcript (+1) encodes the middle surface protein whereas the other two (+20 and +31) can only code for small surface protein. Sequence analysis does not reveal any TATA element. In this study, we employ deletion, linker scanning, and linker insertion analyses to study systematically the sequence requirements for the initiations of all three transcripts and their upstream regulatory sequences. Our study reveals that the sequence downstream of -16 is sufficient for precise initiation of all three groups of S transcripts. The 3' boundary of minimal promoter element is +15 for the +1 transcript, whereas it is +39 for both +20 and +31 transcripts. Furthermore, there are distinct sequence requirements for the initiations of three groups of S transcripts. The sequences from -17 to -10 and from -1 to +7 are required for the initiation of +1 transcript, the sequence from +16 to +39 is essential for the +20 transcript, and the sequences from -17 to -10 and from +24 to +39 are required for the + 31 transcript. Our results also suggest that the transcription initiations of major surface promoter may be mediated in part by initiators. The initiations of these three groups of S transcripts are under differential regulation. The region from -39 to -16 containing both negative and positive regulatory elements selectively regulates the transcription levels of the two major S transcripts. Most notably, mutation of the sequence from -17 to -10, which contains a Sp1 site, leads to an increase in the imprecise initiation at +1 site and depresses the initiation of +20 and, to a greater extent, +31 transcript. The relevance of differential regulation of major surface promoter to the varied production of different surface protein isoforms in viral life cycle is discussed.

A Serine-Kinase-Containing Protein Complex Interacts with the Terminal Protein Domain of Polymerase of Hepatitis B Virus.

Polymerase of human hepatitis B virus is required for viral replication and pregenomic RNA encapsidation. Using recombinant GST fusion proteins, we show that the terminal protein domain of polymerase can interact specifically with a protein complex containing kinase activity and a tightly associated 35-kD protein (p35). This kinase is termed terminal-protein-associated kinase (TPAK). The phosphoamino acid analysis of phosphorylated p35 demonstrates that TPAK is a serine kinase. Analysis of deletion mutants shows that amino acids 1-95 of the terminal protein domain are required for the interaction with TPAK/p35 and phosphorylation of p35. TPAK/p35 are found predominantly in the cytoplasm. Furthermore, TPAK can be inhibited by heparin and manganese ions, but is resistant to spermidine, DRB, H89 or H7. These results indicate that TPAK is not protein kinase A or protein kinase C. Copyright 1997 S. Karger AG, Basel

Correlation between hepatic hepatitis B core antigen and serum hepatitis B virus-DNA levels in patients with chronic hepatitis B virus infections in Taiwan.

Paired liver biopsy specimens and serum samples from 76 patients with chronic hepatitis B virus (HBV) infection were taken for staining of hepatitis B core antigen (HBcAg) by immunoperoxidase and testing of HBV-DNA by a spot hybridization technique, respectively. Thirty-two tissue specimens showed positive staining for HBcAg in their hepatocytes. The two patients with diffuse HBcAg expression in liver tissue also had high serum concentrations of HBV-DNA (greater than 10 pg/10 microL). Among 30 patients with focal HBcAg distribution, 28 patients (93.3%) had measurable levels of serum HBV-DNA and 17 patients (60.7%) had high levels of serum HBV-DNA. Of 44 patients without hepatic HBcAg expression, only 12 patients (27.3%) had detectable serum HBV-DNA, and most patients (93.1% [11/12]) had low concentrations (less than 10 pg/10 microL). Nineteen patients had superimposed hepatitis D virus infection, and, of these, three patients (15.8%) had detectable serum HBV-DNA in low concentrations, while one of the three patients had stainable HBcAg in his hepatocytes with focal distribution. Two of the three patients with hepatitis A virus superinfection who had focal HBcAg expression in their liver tissue had serum HBV-DNA levels that were high during the acute phase of hepatitis A virus infection, and in one patient his serum HBV-DNA levels further increased from 10 pg/10 microL to 40 pg/10 microL during the recovery phase. Thus, measurement of serum HBV-DNA levels in patients with chronic HBV infection correlated well with their hepatic HBcAg expression, and both represent the precise status of HBV replication.

Differentiated liver cell specificity of the second enhancer of hepatitis B virus.

Hepatitis B virus is a hepatotropic virus. Its replication and gene expression are mainly restricted to hepatocytes in the infective process. The viral gene expression thus provides a unique system with which to study the control of tissue-specific gene expression. We have previously reported the identification and characterization of the second enhancer (enhancer II) of hepatitis B virus. In this report, we further demonstrate that the minimal functional constituents of the second enhancer, box alpha and box beta, display liver cell and differentiation state specificity. Moreover, box alpha exhibits the same liver cell and differentiation state specificity when functioning as an upstream regulator for the basal core promoter. Gel shift experiments reveal a unique box alpha-binding protein, protein a, which is present only in differentiated liver cells, where enhancer II is functional. The converse is true for another box alpha-binding protein, protein f, which is present only in poorly differentiated liver cells and nonliver cells. The simplest hypothesis that explains these results is that protein a activates and/or protein f suppresses the enhancer and upstream regulator functions. Although C/EBP is a candidate for a transcription factor that interacts with box alpha or box beta, none of the binding factors identified in the gel shift assays, including protein a and protein f, is likely to be C/EBP because they differ from C/EBP in heat lability and sequence preference.

Repression of enhancer II activity by a negative regulatory element in the hepatitis B virus genome.

Enhancer II of human hepatitis B virus has dual functions in vivo. Located at nucleotides (nt) 1646 to 1741, it can stimulate the surface and X promoters from a downstream position. Moreover, the same sequence can also function as upstream regulatory element that activates the core promoter in a position- and orientation-dependent manner. In this study, we report the identification and characterization of a negative regulatory element (NRE) upstream of enhancer II (nt 1613 to 1636) which can repress both the enhancer and upstream stimulatory function of the enhancer II sequence in differentiated liver cells. This NRE has marginal inhibitory effect by itself but a strong repressive function in the presence of a functional enhancer II. Mutational analysis reveals that sequence from nt 1616 to 1621 is required for repression of enhancer activity by the NRE. Gel shift analysis reveals that this negative regulatory region can be recognized by a specific protein factor(s) present at the 0.4 M NaCl fraction of HepG2 nuclear extracts. The discovery of the NRE indicates that HBV gene transcription is controlled by combined effects of both positive and negative regulation. It also provides a unique system with which to study the mechanism of negative regulation of gene expression.

Effect of phosphate and adenine nucleotides on the rate of labeling of functional groups at the catalytic site of F1-ATPase.

The effect of inorganic phosphate, ADP, ATP, and their analogues on the rate of labeling of F1-ATPase by 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) and phenylglyoxal have been investigated. Analysis of the kinetic data indicate that the labeled functional groups of the essential tyrosine and arginine residues respectively are both located at the catalytic site of F1. The active phenolic group of tyrosine is located closer to the bound inorganic phosphate or the gamma-phosphate group than the alpha- and beta-phosphate groups of the bound ATP at the catalytic site, whereas the guanidinium group of arginine is located closer to the alpha- and beta-phosphate groups of the bound ATP than to its gamma-phosphate group or the bound inorganic phosphate. The kinetically deduced dissociation constants are 1.3 mM and 210 microM for the inorganic phosphate and aDP respectively bound to this catalytic site. Labeling the essential tyrosine residue by NBD-Cl has been found to facilitate subsequent labeling of the essential arginine residue by phenylglyoxal.

Functional groups at the catalytic site of F1 adenosine triphosphatase.

The protection of F1 ATPase by inorganic phosphate, ADP, ATP, and magnesium ion against inactivation by 1-fluoro-2,4-dinitrobenzene, 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole, and 1-(ethoxycarbonyl)-2-ethoxy-1,2-dihydroquinoline, respectively, has been investigated. Dissociation equilibrium constants and rate constants for the labeling reactions have been deduced from a quantitative treatment of the kinetic data. Comparison of these dissociation constants with each other and with the corresponding literature values indicates that the essential Tyr, Arg, Lys, and Glu or Asp residues are indeed located at the catalytic site of the enzyme. Examination of the rate constants for the labeling reactions in the presence of excess inorganic phosphate, ADP, ATP, or magnesium ion, respectively, suggests that the essential phenol and amino groups are located nearer to the bound inorganic phosphate or the gamma-phosphate group than to the alpha- or beta-phosphate group of the bound ATP, that the essential guanidinium group is located nearer to the alpha- or beta-phosphate group than to the gamma-phosphate group of the bound ATP or the bound inorganic phosphate, and that the essential carboxylate group is located slightly farther away but complexed with magnesium ion which it shares with the bound inorganic phosphate. A mechanism consistent with these topographical relationships is proposed for the catalytic hydrolysis and synthesis of ATP.

Transcriptional repression of human hepatitis B virus genes by a bZIP family member, E4BP4.

Box alpha is an essential element of both the upstream regulatory sequence of the core promoter and the second enhancer, which positively regulate the transcription of human hepatitis B virus (HBV) genes. In this paper, we describe the cloning and characterization of a box alpha binding protein, E4BP4. E4BP4 is a bZIP type of transcription factor. Overexpression of E4BP4 represses the stimulating activity of box alpha in the upstream regulatory sequence of the core promoter and the second enhancer in differentiated human hepatoma cell lines. E4BP4 can also suppress the transcription of HBV genes and the production of HBV virions in a transient-transfection system that mimics the viral infection in vivo. Expression of an E4BP4 antisense transcript can, instead, elevate the transcription of the core promoter. A low abundance of E4BP4 protein and mRNA in differentiated human hepatoma cell lines is detected, and E4BP4 is not a major component of box alpha binding proteins in untransfected differentiated human hepatoma cell lines. C/EBPalpha and C/EBPbeta, in contrast, are major components of the box alpha binding activity present in nuclear extracts. E4BP4 has a stronger binding affinity towards box alpha than the endogenous box alpha binding activity present in nuclear extracts. Structure and function analysis of E4BP4 reveals that DNA binding activity is sufficient to confer the negative regulatory function of E4BP4. These results indicate that binding site occlusion is the mechanism whereby E4BP4 suppresses transcription in HBV.

Phosphorylation of the core protein of hepatitis B virus by a 46-kilodalton serine kinase.

Core protein is the major component of the core particle (nucleocapsid) of human hepatitis B virus. Core particles and core proteins are involved in a number of important functions in the replication cycle of the virus, including RNA packaging, DNA synthesis, and recognition of viral envelope proteins. Core protein is a phosphoprotein with most, if not all, of the phosphorylation on C-terminal serine residues. In this study, we identified a serine kinase activity from the ribosome-associated protein fraction of cytoplasm that could specifically bind and phosphorylate the C-terminal portion of recombinant core protein. This kinase is referred to as core-associated kinase (CAK). CAK could be inhibited by the kinase inhibitors heparin and manganese ions but not by spermidine, DRB, H89, or H7, indicating that CAK is distinct from protein kinase A and protein kinase C. CAK could be partially purified by heparin-Sepharose CL-6B and phosphocellulose P11 columns. By using a far-Western assay, three specific proteins, of 46, 35, and 13 kDa, were shown to interact with the C-terminal part of the core protein. These three proteins were present only in the eluted fractions that contains the CAK activity. An in-gel kinase assay showed that a 46-kDa kinase in the same fraction could bind and phosphorylate the C-terminal part of the recombinant core protein. These results indicate that this 46-kDa kinase is most probably CAK. A similar 46-kDa kinase, which exhibits the same profile of sensitivity to kinase inhibitors as that of CAK, is present in both purified intracellular core particles and extracellular 42-nm virions, suggesting that CAK is a candidate for the core particle-associated kinase.

Functional groups at the catalytic site of BF1 adenosinetriphosphatase from Escherichia coli.

The rates of inactivation of BF1 adenosinetriphosphatase (BF1-ATPase) from Escherichia coli by 7-chloro-4-nitro-2,1,3-benzoxadiazole, 1-fluoro-2,4-dinitrobenzene, 2,4,6-trinitrobenzenesulfonate, phenylglyoxal, and N,N'-dicyclohexylcarbodiimide have been measured in the presence and absence of various concentrations of inorganic phosphate, ADP, ATP, or magnesium ion. Dissociation equilibrium constants and rate constants for the labeling reactions have been deduced from a quantitative treatment of the kinetic data. The results suggest that the essential Tyr, Lys, Arg, and Glu or Asp residues are probably located at the catalytic site of BF1-ATPase and that in addition to steric interference, the effect of charge interaction should also be considered in interpreting the kinetic data on the protection of BF1-ATPase by substrate molecules against inactivation by the above labeling reagents. Examination of the relative values of the rate constants for the labeling reactions in the presence and absence of inorganic phosphate, ADP, ATP, or magnesium ion, respectively, and the effect of NBD label on the rates of labeling of the essential guanidinium, amino, and carboxyl groups suggest that the arrangement of these four functional groups at the catalytic site of BF1 may be similar to that previously proposed for MF1-ATPase from bovine heart; namely, the essential amino group and the unusually reactive phenol group are probably located near the bound inorganic phosphate or the gamma-phosphate group of the bound ATP, the essential guanidinium group is probably located nearer to the alpha- or beta-phosphate group than to the gamma-phosphate group of the bound ATP or the bound inorganic phosphate, and the essential carboxylate group is probably complexed with a magnesium ion which it shares with the bound inorganic phosphate.

Characterization of nucleotide sequence of hepatitis B surface gene and enhancer element of HBV.

The HBV DNA sequence containing a primary part of pre-S2, the coding sequence of the major S protein and the enhancer sequence in pTWS1 was completely sequenced. A comparison with other previously published HBV sequences indicates that the nucleotide sequence homology in the pre-S2 region is 97%, 98% and 98% for adw, adr and ayw subtypes, respectively, and 76% among different subtypes. The nucleotide sequence homology in the major S protein region is 99% for each adw, adr and ayw subtype, and 90% among different subtypes. The important regulatory enhancer sequence (nucleotides 1036-1204) is identical in all subtypes.

The genome of hepatitis B virus contains a second enhancer: cooperation of two elements within this enhancer is required for its function.

Previous studies have identified an enhancer (enhancer I) at nucleotides (nt) 1074 to 1234 in the genome of the human hepatitis B virus (HBV), which locates immediately upstream from the X gene. By analysis of the expression of the chloramphenicol acetyltransferase gene driven by a heterologous simian virus 40 early promoter, we describe the identification of a second enhancer (enhancer II) at nt 1636 to 1741, which locates downstream of enhancer I and immediately upstream of the core gene. With various deletions at the 5' end of enhancer II, a positive regulatory element was identified at nt 1636 to 1690 (the II-A element), with the 5' boundary between nt 1636 and 1671. The II-A element alone did not have an enhancer function, but the enhancer activity was achieved by the concomitant presence of the sequence from nt 1704 to 1741 (the II-B element). The II-B element alone did not have enhancer activity. These results indicate that cooperation between the II-A and II-B elements is required to exhibit the enhancer activity of enhancer II. We also show that enhancer II stimulates the transcriptional activity of both the SPI and SPII promoters of the surface gene. Therefore, the SPI promoter activity is regulated by the proximal HNF-1 binding element and the distal enhancers I and II. These results indicate that multiple regulatory elements scattered over the whole viral genome are involved in the regulation of expression of each individual HBV gene and that the same regulatory element controls the expression of different HBV genes. The relative positions of these regulatory elements in the HBV genome suggest that they may control the expression of HBV genes in a coordinate and cooperative manner.

The surface gene promoter of the human hepatitis B virus displays a preference for differentiated hepatocytes.

By transfections of hepatitis B virus (HBV) DNA into five human hepatoma cell lines with the characteristics of differentiated human hepatocytes, three human hepatoma cell lines possessing partial hepatocyte-associated markers, and one non-liver cell line, we demonstrated that the expression of hepatitis B surface and core genes preferentially occurred in hepatoma cell lines with differentiated hepatocyte-associated characteristics. With a heterologous CAT gene as a reporter, the transcriptional activity of the promoter region containing both the distal (SPI) and the proximal (SPII) promoters of hepatitis B surface gene was found to show a preference for differentiated hepatoma cell lines. The SPI promoter which produces a RNA transcript for the synthesis of the large surface protein shows a strong preference, at least 750-fold, for differentiated hepatoma cells, while the SPII promoter which produces RNA transcripts for the synthesis of the middle and major surface proteins shows a moderate preference, about 20- to 59-fold. Further study indicates that this 750-fold preference of the SPI transcriptional activity for differentiated hepatoma cell lines can be attributed to the regulatory sequences of both the SPI and the HBV enhancer regions. These results also imply the important role of the large surface protein of HBV on the hepatocyte-specific infectivity of this virus.

Hepatitis B virus core protein interacts with the C-terminal region of actin-binding protein.

Hepatitis B viral core protein is present in the nucleus and cytoplasm of infected hepatocytes. There is a strong correlation between the intrahepatic distribution of core protein and the viral replication state and disease activity in patients with chronic hepatitis. To understand the role of core protein in the pathogenesis of HBV, we used a yeast two-hybrid system to search for cellular proteins interacting with the carboxyl terminus of core protein, as this region is involved in a number of important functions in the viral replication cycle including RNA packaging and DNA synthesis. A cDNA encoding the extreme C-terminal region of human actin-binding protein, ABP-276/278, was identified. This interaction was further confirmed both in vitro and in vivo. In addition, the extreme C-terminal region of ABP-276/278 interacted with the nearly full-length HBV core protein. Since this region is present in both the core and the precore proteins, it is likely that both core and precore proteins of HBV can interact with the C-terminal region of ABP-276/278. The minimal region of ABP-276/278 which interacted with the HBV core protein was the C-terminal 199 amino acid residues which correspond to part of the 23rd repeat, the entire 24th repeat and the intervening hinge II region in ABPs. The potential functional outcome of ABP interaction in HBV replication and its contribution to the pathological changes seen in patients with chronic HBV infection are discussed.

Overlapping initiator and TATA box functions in the basal core promoter of hepatitis B virus.

The regulation of transcription of the hepatitis B virus core promoter is an important event in the viral life cycle. Two messages, precore and pregenomic RNAs, that are initiated 30 nucleotides apart are produced by the core promoter. Precore RNA encodes nucleocapsid protein and pregenomic RNA core and polymerase. The latter transcript also serves as a template for viral genome replication via reverse transcription. We have previously defined a basal core promoter, which contains four TA-rich sequences (TA1 through TA4) but no canonical TATA element, that can direct transcription of both messages. In this study, we demonstrated that a stretch of 15 nucleotides containing TA4 is sufficient to direct precise initiation of both precore and pregenomic transcripts. This sequence can function as both an initiator and a TATA element. Mutational analysis further revealed that sequences essential for either function are colocalized. The significance of this finding with respect to the basal transcription mechanism and regulation of viral gene expression is discussed.