No abstract

The human papillomavirus (HPV) E2 protein regulates viral gene expression and is also required for viral replication. HPV-transformed cells often contain chromosomally integrated copies of the HPV genome in which the viral E2 gene is disrupted. We have shown previously that re-expression of the HPV 16 E2 protein in HPV 16-transformed cells results in cell death via apoptosis. Here we show that the HPV 16 E2 protein can induce apoptosis in both HPV-transformed and non-HPV-transformed cell lines. E2-induced apoptosis is abrogated by a trans-dominant negative mutant of p53 or by overexpression of the HPV 16 E6 protein, but is increased by overexpression of wild-type p53. We show that mutations that block the DNA binding activity of E2 do not impair the ability of this protein to induce apoptosis. In contrast, removal of both N-terminal domains from the E2 dimer completely blocks E2-induced cell death. Heterodimers formed between wild-type E2 and N-terminally deleted E2 proteins also fail to induce cell death. Our data suggest that neither the DNA binding activity of E2 nor other HPV proteins are required for the induction of apoptosis by E2 and that E2-induced cell death occurs via a p53-dependent pathway.Papillomaviruses infect epithelial cells and generally induce the formation of benign hyperproliferative lesions. However, some papillomavirus types are associated with cancer. For example, human papillomavirus (HPV) 1 types 16 and 18 have been linked to cervical cancer in women (1) and bovine papillomavirus (BPV) types 2 and 4 have been linked to bladder cancer and cancer of the upper alimentary canal respectively, in cattle (2, 3). Human cervical cancers express the viral E6 and E7 oncogenes, and the products of these genes increase cell proliferation and promote cell immortalization (for a review, see Ref. 4). The human papillomavirus E2 gene, or lack thereof, is also thought to play a major role in the development of cervical cancer. Most cervical cancers contain chromosomally integrated copies of the HPV genome in which the viral E2 gene has been disrupted (5). Furthermore, mutations in the E2 gene increase the immortalization capacity of HPV 16 (6).The papillomavirus E2 genes encode sequence-specific DNAbinding proteins that regulate viral gene expression and are also required for viral DNA replication (reviewed in Ref. 7). The E2 proteins bind as dimers to multiple copies of an inverted repeat sequence found within the viral long control region. Depending on the particular virus and the particular E2 protein being studied, the binding of E2 to these sites can either activate or repress transcription of the E6 and E7 oncogenes. For example, the HPV 16 E2 protein activates transcription from the P97 promoter located at the 3Ј end of the HPV 16 long control region, whereas, under exactly the same conditions, the BPV1 E2 protein represses P97 promoter activity (8,9). Each subunit of the E2 dimer contains two domains separated by a flexible hinge: the N-terminal domain of each subunit mediates the regulation o...

The effects of a number of mutations in the E. coli cyclic AMP receptor protein (CRP) have been determined by monitoring the in vivo expression and in vitro open complex formation at two semi-synthetic promoters that are totally CRP-dependent. At one promoter the CRP-binding site is centered around 41.5 base pairs upstream from the transcription start whilst at the other promoter it is 61.5 base pairs upstream. The CRP mutation E171K reduces expression from both promoters whilst H159L renders CRP totally inactive: neither mutation stops CRP binding at either promoter. The mutations K52N and K52Q reverse the effect of H159L and 'reeducate' CRP to activate transcription. CRP carrying both H159L and K52N activates transcription from the promoter with the CRP site at -41.5 better than wild type CRP. In sharp contrast, this doubly changed CRP is totally inactive with respect to the activation of transcription from the promoter carrying the CRP site at -61.5. Our results suggest that CRP can use different contacts and/or conformations during transcription activation at promoters with different architectures.

For many, if not most genes, the initiation of transcription is the principle point at which their expression is regulated. Transcription factors, some of which bind to specific DNA sequences, generally either activate or repress promoter activity and thereby control transcription initiation. Recent work has revealed in molecular detail some of the mechanisms used by transcription factors to bring about transcriptional repression. Some transcriptional repressor proteins counteract the activity of positively acting transcription factors. Other repressors inhibit the basal transcription machinery. In addition, the repression of transcription is often intimately associated with chromatin re-organisation. Many transcriptional repressor proteins interact either directly or indirectly with proteins that remodel chromatin or can themselves influence chromatin structure. This review discusses the mechanisms by which transcriptional repression is achieved and the role that chromatin re-organisation plays in this process.