Figure 7.

Sequence-dependent site exposure in nucleosome. (A) MNase digestion on preferential sequence. When the preferential sequence (e.g. TATA) is at the end region where MNase can access from the 5′ end of DNA, TATA would be favourably cleaved. However, if it is at the internal region where TATA is tightly bound on histones, cleavages are prohibited. (B) MNase digestion on site-exposure sequence. When the site-exposure sequence (e.g. AAAA) is at the end region, because MNase can access from the 5′ end of DNA and sequence-dependent site exposure occurs, cleavages on AAAA are allowed, though less favourably than TATA. When it is at the internal site, due to site exposure, cleavages will also occur. (C) When multiple sites composed of the site-exposure sequence are assembled at one end of nucleosome (i.e. DNA entry site), the overall affinities between DNA and histones will dwindle to assist nucleosome unwrapping with the presence of transcription factors or chromatin remodellers (shown in green ellipse).