Nucleoprotein complexes containing viral DNA and cellular histones were extracted from nuclei of permissive cells infected with polyoma virus or simian virus 40 (SV40) and examined by electron microscopy. Polyoma and SV40 nucleoprotein complexes are almost identical. They appear as relaxed circular molecules consisting of 20 to 21 globular particles interconnected by thin filaments. Their contour length in 0.02 M salt is 2.7 times shorter than that of viral DNA form I obtained after dissociation of the proteins in 1 M NaCl. The nucleosomes have an average diameter of 12.5 nm. Each nucleosome contains 175 to 205 DNA base pairs condensed fivefold in length. The nucleosomes are regularly spaced on the circular molecule. The internucleosomal filaments are made of naked DNA, and each filament contains about 55 base pairs. The partial sensitivity of the nucleoprotein complex to cleavage by EcoRl endonuclease suggests that the nucleosomes are not formed at specific sites on the viral genome. Faster sedimenting nucleoprotein complexes containing replicative intermediates were studied. Isopycnic centrifugation in metrizamide gradients in the absence of aldehyde fixation showed that these molecules conserved the same DNA-toprotein ratio as the form I DNA-containing complexes.
The retinoblastoma (RB) gene product has been shown to restrict cell proliferation, promote cell differentiation, and inhibit apoptosis. Loss of RB function can induce both p53-dependent apoptosis and p53-independent apoptosis; little is known about the mechanisms of RB-regulated p53-independent apoptosis. Here we show that RB specifically activates transcription of the survival gene bcl-2 in epithelial cells but not in NIH 3T3 mesenchymal cells. This transcriptional activity is mediated by the transcription factor AP-2. By monitoring protein-DNA interactions in living cells using formaldehyde cross-linking and chromatin immunoprecipitation, we show that endogenous RB and AP-2 both bind to the same bcl-2 promoter sequence. In addition, we demonstrate that RB and AP-2 also bind to the E-cadherin gene promoter in vivo, consistent with regulation of this promoter by both AP-2 and RB in epithelial cells. This study provides evidence that RB activates bcl-2 and E-cadherin by binding directly to the respective promoter sequences and not indirectly by repressing an inhibitor. This recruitment is mediated by a transcription factor, in this case AP-2. For the first time, our results suggest a direct molecular mechanism by which RB might inhibit apoptosis independently of p53. The results are discussed in a context where RB and Bcl-2 contribute under nonpathological conditions to the maintenance of cell viability in association with a differentiated phenotype, contributing to the tumor suppressor function of RB and playing important roles in normal development.The harmonious development of any organism requires a fine equilibrium between the processes of cell proliferation, differentiation, and apoptosis. These different functions are highly regulated and interdependent. Many of the genes involved in the regulation of these functions function as oncogenes or tumor suppressor genes.The retinoblastoma (RB) gene is an important tumor suppressor, and its protein product has been shown to restrict cell proliferation, promote cell differentiation, and inhibit apoptosis (for reviews, see references 15, 28, and 53).RB can act as either a negative or a positive regulator of transcription. In the context of cell proliferation, it acts as a negative regulator. The most widely accepted hypothesis proposes that RB represses transcription through the E2F family of transcription factors, partly through masking their activation domains and partly by recruiting a histone deacetylase to promoters that are repressed during the G 1 phase of the cell cycle (for a review, see reference 37). It has also been suggested that repression and activation of E2F-responsive genes may occur through distinct E2F heterodimers (46). In contrast to cell proliferation, when RB promotes differentiation, it regulates the activity of several transcription factors in a positive manner (for a review, see reference 35). However, the precise molecular mechanism for this activity of RB has not yet been elucidated. Similarly, RB also inhibits apoptosis, but it is not...
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