Negative constrained DNA supercoiling in archaeal nucleosomes.

Archaeal histones have significant sequence and structural similarity to their eukaryal counterparts. However, whereas DNA is wrapped in negatively constrained supercoils in eukaryal nucleosomes, it has been reported that DNA is positively supercoiled by archaeal nucleosomes. This was inferred from experiments performed at low temperature and low salt concentrations, conditions markedly different from those expected for many archaea in vivo. Here, we report that the archaeal histones HMf and HTz wrap DNA in negatively constrained supercoils in buffers containing potassium glutamate (K-Glu) above 300 mM, either at 37 degrees C or at 70 degrees C. This suggests that high salt concentrations allow an alternate archaeal nucleosome topology: a left-handed tetramer rather than the right-handed tetramer seen in low salt conditions. In contrast, the archaeal histone MkaH produces DNA negative supercoiling at all salt concentrations, suggesting that this duality of structure is not possible for this atypical protein, which is formed by the association of two histone folds in a single polypeptide. These results extend the already remarkable similarity between archaeal and eukaryal nucleosomes, as it has been recently shown that DNA can be wrapped into either positive or negative supercoils around the H3/H4 tetramer. Negative supercoiling could correspond to the predominant physiological mode of DNA supercoiling in archaeal nucleosomes.