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Biomedical subjects

A Ori

Publications and source records attributed to A Ori.

At least 19 recordsLinked to original sources
PubMed2000

A composite polyadenylation signal with TATA box function.

A variant polyadenylation signal, which is conserved and employed by mammalian hepadnaviruses, has a sequence resembling that of the TATA box. We report here that this composite box manifests all the promoter characteristics. It binds effectively TATA-binding protein with TFIIB and TFIIA in a synergistic manner. This capacity, however, is lost when the box is converted to a canonical and simple poly(A) signal. Furthermore, we show that it has promoter activity and supports transcription of reporter genes preferentially in liver-derived cells, a characteristic behavior of the hepatitis B virus (HBV) promoters. In addition, we show that the HBV noncanonical poly(A) signal supports transcription initiation from the viral genome, suggesting that it is a genuine promoter, possibly of the polymerase/reverse transcriptase gene. Finally, we found that this deviant poly(A) signal is crucial for HBV replication since a viral mutant with a canonical poly(A) box is impaired in replication. Our data, therefore, raise the interesting and novel possibility that a composite poly(A) box might have a dual function. At the level of DNA it functions as a promoter to initiate transcription, whereas at the level of RNA it serves as a poly(A) signal to process RNA. An interesting outcome of this strategy of gene expression is that it provides a novel mechanism for the synthesis of an approximately genome length transcript.

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PubMed1998

p53 binds and represses the HBV enhancer: an adjacent enhancer element can reverse the transcription effect of p53.

The transcription program of the hepatitis B virus (HBV) genome is regulated by an enhancer element that binds multiple ubiquitous and liver-enriched transcription activators. HBV transcription and replication are repressed in the presence of p53. Here we describe a novel molecular mechanism that is responsible for this repression. The p53 protein binds to a defined region within the HBV enhancer in a sequence-specific manner, and this, surprisingly, results in p53-dependent transcriptional repression in the context of the whole HBV enhancer. This unusual behavior of the HBV enhancer can be reconstituted by replacing its p53-binding region with the p53-binding domain of the mdm2 promoter. Remarkably, mutation of the EP element of the enhancer reversed the effect of p53 from repression to transcriptional stimulation. Furthermore, EP-dependent modulation of p53 activity can be demonstrated in the context of the mdm2 promoter, suggesting that EP is not only required but is also sufficient to convert p53 activity from positive to negative. Our results imply that the transcriptional effect of DNA-bound p53 can be dramatically modulated by the DNA context and by adjacent DNA-protein interactions.

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PubMed1995

Hepatitis B virus enhancer binds and is activated by the Hepatocyte nuclear factor 3.

The enhancer of hepatitis B (HBV) virus displays a liver-specific activity that determines the postreceptor virus-host tropism. Despite the detailed study of this enhancer our knowledge of the mechanisms underlying this behavior is very limited. Here we report that the hepatocyte nuclear factor 3 (HNF3) is at least in part responsible for the liver-specific activity of the enhancer. We demonstrate that recombinant HNF3 alpha binds the enhancer at two sites with different affinity. Transfection studies have demonstrated that the enhancer is active only in liver cells and that integrity of the HNF3 binding sites is important for its full activity. In vitro transcription assays revealed that the enhancer is active only in liver extracts but not in extracts prepared from HeLa cells. Furthermore, the latter extract cannot be activated by addition of recombinant HNF3 alpha. A similar behavior is manifested in transfected cells and, here again, the inactive enhancer is not activated by cotransfected HNF3 beta and alpha. Collectively, our study shows that HNF3 activators are required but not sufficient for full activation of the HBV enhancer and there is a need for additional liver-specific activators or coactivators.

Base Sequence↗
PubMed1994

An NF1 motif plays a central role in hepatitis B virus enhancer.

The hepatitis B virus enhancer plays an important role in transcription regulation of the viral genes in a liver-specific manner. In animal models a homologous element seems to be involved in activation of cellular oncogenes and tumorigenesis. Previously, the enhancer was divided into several functional domains, whereby each one seemed to be required for optimal transcription activity. To gain more information on the mode of action of these elements and their role in viral genome, we mutagenized the individual enhancer elements and analyzed their functions in three different experimental systems. All show that the NF1b motif of the enhancer plays a central role, with the most dramatic results obtained from the cell-free in vitro transcription assay. Furthermore, an intact viral genome mutated at the NF1b site is a poor template for the synthesis of the 3.5-kb pregenomic RNA. These data are rather unexpected, given the ubiquitous appearance of this factor. On the other hand, our findings are in agreement with a large number of recently reported cases in which NF1 seems to determine tissue-specific expression of a wide range of cellular and viral promoters.

Animals↗