Structure of the core histones

Although the histone fold was first described from the structure of the histone octamer core of the nucleosome [17], the high a-helical content was predicted much earlier [43]. The core histones possess three functional domains (1) the histone fold domain, (2) an N-terminal tail domain, and (3) various accessory helices and less structured regions. The N-terminal tail domains of the core histones are currently the focus of intense research. Covalent modifications of residues in these unstructured domains appear to modify local chromatin structure, either directly or 

The histone fold appears as a symmetrical duplication of a helix-fold-helix motif [18,19] with a long median helix, the mH helix, and two shorter terminal helices, the N-terminal or NH helix and the C-terminal or CH helix (Fig. 7). The helices are joined by loops N-terminal, the NL loop, and C-terminal, the CL loop, to the mH helix. The formation of heterodimer pairs is accomplished through a handshake [17] pairing in which the mH helices of the handshake partners align in opposite orientations such that the NL loop of one partner aligns in a parallel orientation with the CL loop of the other (Fig. 8). 

75 superhelical turns of DNA on the NCP are the longest DNA sequences to date whose structure has been experimentally determined. Although the DNA is B-form, deviations from ideality exist. Wrapping of the DNA onto the 

The center of the palindromic DNA was of necessity changed from the native a-satellite to provide for construction of the palindrome [30]. The deviation of the palindrome from perfect symmetry on the histone octamer may be a 

The a-mGI motif operates primarily through the induced dipole of the a helices directed toward DNA phosphates (Fig. 13a) and was termed the paired element motif (PEM) [17], although paired ends of helices is more descriptive. The guanido group is restrained from binding deeply into the mGI by a threonine from another histone. This steric hindrance preserves the avidity of the histone core for DNA, but prevents overly strong binding that would disrupt the plasticity of the histone-DNA interaction. 

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