Structural properties of yeast chromatin fiber examined by AFM and in vitro reconstitution system

The most fundamental structure of the chromosome is nucleosome, which forms “beads-on-a-string” structure with the width of 11 nm. The nucleosomal array is folded into higher-order structures with the help of linker histones, non-histone chromosomal proteins and histone-tails. This study analyzed the structure of reconstituted chromatin using S. pombe (fission yeast) core histones. Histone H2B of fission yeast has less lysine residues than that of higher eukaryotes and no linker histone gene has been found in the genome. To examine how the structural property of yeast chromatin differs from that of higher eukaryotes, fission yeast core histones were purified and used in chromatin reconstitution. In low salt buffer (50 mM NaCl), yeast and HeLa nucleosomal arrays showed similar “beads-on-a-string” structures. How ever, in higher salt buffer (100 and 200 mM NaCl), HeLa nucleosomal arrays formed aggregates, while yeast nucleosomal arrays did not. Hyper-acetylated HeLa nucleosomal array also failed to form aggregates in higher salt conditions. Similar to HeLa nucleosomes, yeast nucleosomal array also formed a thicker fiber structures upon the addition of linker histone H1. These results suggest that the yeast nucleosomal array possesses similar structural properties to the hyper-acetylated nucleosomal array of higher eukaryotes, possibly due to the comparable electric charge in the tail regions.