Nucleosome fixation

The structure of the promoter region of some of the genes studied overlaps a nucleosome in such a way that the RNA polymerase II cannot get to its binding site. The interaction between the receptor dimers and the HRE causes the nucleosomes to have their structures altered, either by being displaced or by having their structure come partly undone. This change in the configuration permits the fixation of the RNA polymerase II, with which the transcription can be initiated. 

The poor resolution of the fine structures of DNA and nucleosomes by conventional SEM was solved by the development of an ultrahigh-resolution SEM (14-15). Inaga et al. (16) observed the DNA double helix and nucleosomes of chicken erythrocytes by using an ultrahigh-resolution SEM. They modified the microspreading technique of Seki et al. (17) and combined it with the carbon plate method devised by Tanaka et al. (18). Briefly, they (procedures were performed at 0-4°C before fixation with the formalin solution) ... 

Fig. 3. Binding of HMf protein from Methanothermus fervidus to pUC18-DNA. (Adapted from ref [23].) Panel A shows the stronger binding of HMf to super-coiled DNA compared to open circular or linear DNA. Panel B shows the small loops found with unfixed material. Panel C shows the formation of nucleosome-like structures on linearized fragments with fixation of glutaraldehyde. The bar in panel B represents 0.5 pm. Courtesy R. Lurz.

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