Atomic force microscopy for chromatin structure study

In eukaryotes, DNA is packed in a highly condensed and hierarchically organized structure called chromatin, in which DNA tightly wraps around the histone octamer consisting of one histone 3-histone 4 (H3-H4) tetramer and two histone 2A- histone 2B (H2A-H2B) dimers with 147 base pairs in an almost two left handed turns. Almost all DNA dependent cellular processes, such as DNA duplication, transcription, DNA repair and recombination, take place in the chromatin form. Based on the critical importance of appropriate chromatin condensation, this thesis focused on the folding behavior of the nucleosome array reconstituted using different templates with various controllable factors such as histone tail modification, linker DNA length, and DNA binding proteins. Firstly, the folding behaviors of wild type (WT) and nucleosome arrays reconstituted with acetylation on the histone H4 at lysine 16 (H4K16 (Ac)) were studied. In contrast to the sedimentation result, atomic force microscopy (AFM) measurements revealed no apparent difference in the compact nucleosome arrays between WT and H4K16 (Ac) and WT. Instead, an optimal loading of nucleosome along the template was found necessary for the Mg2+ induced nucleosome array compaction. This finding leads to the further study on the role of linker DNA in the nucleosome compaction. A method of constructing DNA templates with varied linker DNA lengths was developed, and uniformly and randomly spaced nucleosome arrays with average linker DNA lengths of 30 bp and 60 bp were constructed. After comprehensive analyses of the nucleosome arrays' structure in mica surface, the lengths of the linker DNA were found playing an important role in controlling the structural geometries of nucleosome arrays in both their extended and compact forms. In addition, higher concentration of the DNA binding domain of the telomere repeat factor 2 (TRF2) was found to stimulate the compaction of the telomeric nucleosome array. Finally, AFM was successfully applied to investigate the nucleosome positioning behaviors on the Mouse Mammary Tumor Virus (MMTV) promoter region, and two highly positioned region corresponded to nucleosome A and B were identified by this method.