Transcriptional silencing in mammals is often associated with promoter methylation. However, a considerable number of genomic methylated CpGs exist in transposable elements, which are frequently found in intronic regions. To determine whether intragenic methylation influences transcription efficiency, we used the Cre/loxP-based system, RMCE, to introduce a transgene, methylated exclusively in a region downstream of the promoter, into a specific genomic site. This methylation pattern was maintained in vivo, and yielded a clear decrease in transgene expression relative to an unmethylated control. Notably, RNA polymerase II (Pol II) was depleted exclusively in the methylated region, as was histone H3 di- and trimethylated on Lys4 and acetylated on Lys9 and Lys14. As the methylated region adopts a closed chromatin structure in vivo, we propose that dense intragenic DNA methylation in mammalian cells initiates formation of a chromatin structure that reduces the efficiency of Pol II elongation.
The surface plasmon damping induced by high excitation of the electron gas is studied in femtosecond pump-and-probe experiments on gold colloids embedded in a sol-gel matrix. Optical excitation of single-particle interband transitions leads to a pronounced broadening of the surface plasmon line. A similar behavior is observed for resonant excitation of the surface plasmon. This broadening is the dominant optical nonlinearity of the system, and reflects the excitation-induced damping of the surface plasmon resonance. The time evolution of the damping rate follows that of the electronic scattering rate.
Abstract. Ag colloid-containing coatings on soda lime glass and fused silica are prepared via the sol-gel process. To incorporate Ag +-ions in the coatings homogeneously, they are stabilized by a functionalised silane (aminosilane) and then mixed with the basic sol prepared from 3-glycidoxypropyl trimethoxysilane (GPTS) and tetraethoxysilane (TEOS). Crack-free and transparent coatings with a thickness of 0.5 to 1.2/zm, are obtained by heat treatment between 120~ and 600~ The Ag-colloid formation was monitored by UV-VIS spectroscopy as a function of temperature. The investigations reveal that the substrate has a deciding influence on the Ag-colloid formation caused by alkali diffusion from the substrate into the coating. High resolution transmission electron microscopy (HRTEM) investigations prove that poly-crystalline AgxOy-nanoparticles are formed during thermal densification in the coatings and that this change is accompanied by a vanishing of the yellow colour of the coatings. A post-heat treatment in a reducing atmosphere (90% N2, 10% Ha) turns back the yellow colour and single-crystalline Agcolloids can be detected by HRTEM. A suitable choice of the temperature and time conditions allows the control of the colloid size during heat treatment in a reducing atmosphere. For comparison, ion-exchange experiments have been carried out which showed that a spontaneous Ag-colloid formation was achieved in the soda lime substrate at 400~ Since Ag containing SiO2-coatings remained colourless after thermal treatment between 400~ and 600~ in air, on soda lime substrates, a remarkable diffusion of Ag + into the substrate was excluded.
Small‐angle X‐ray scattering (SAXS) is generally not applicable to thin (~ 1 μm) films and coatings because of their weak scattering signal in conventional transmission experiments. This restriction can be overcome by working in grazing incidence, with an angle of incidence α slightly above the critical angle of total reflection ϕc, in order to augment the effective X‐ray path in the sample. The possibilities of this technique are demonstrated by preliminary results obtained on glass‐like sol‐gel coatings containing nano‐scaled gold colloids. The scattering of the gold colloids was identified by variations of the X‐ray energy near the absorption edge of gold (anomalous SAXS). It showed the theoretically predicted variation of intensity with energy. The recorded scattering spectra were corrected for absorption and refraction effects, whose influences on the spectra are discussed. Calculated size distributions of the colloids from SAXS in grazing incidence were compared with results from high‐resolution transmission electron microscopy (HTEM) investigations and optical UV‐visible spectroscopy.
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