The traditional view of p53 activation includes three steps—p53 stabilization, DNA binding, and transcriptional activation. However, recent studies indicate that each step of p53 activation is more complex than originally anticipated. Moreover, both genetic studies in mice and in vitro studies with purified components suggest that the classical model may not be sufficient to explain all aspects of p53 activation in vivo. To reconcile these differences, we propose that antirepression, the release of p53 from repression by factors such as Mdm2 and MdmX, is a key step in the physiological activation of p53.
The microstructure of the free volume and its temperature dependence were studied for amorphous Teflon AF 1600 (T g ) 160 °C) and AF 2400 (T g ) 240 °C). These copolymers consist of tetrafluoroethylene and 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole units and are used as highly permeable membrane materials. We employed positron annihilation lifetime spectroscopy (PALS) to investigate the free volume. From the lifetime spectra analyzed with the routine LT9.0 the hole size distribution, its mean size and width were calculated. Results obtained from a three and a four component analysis of lifetime spectra are compared. Both polymers show very large orthopositronium lifetimes τ 3 (corresponding to large hole sizes) and glass transitions in τ 3 near the expected values. AF 2400 shows an unusual behavior characterized by a nonlinear temperature dependence of the orthopositronium lifetime, extraordinarily large mean values of τ 3 and the widths of the lifetime distributions σ 3 . Possible explanations for this will be discussed in relation to the microstructure of the copolymer.
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