DNA-BINDING BY THE ARCHAEAL HISTONE HMF RESULTS IN POSITIVE SUPERCOILING

HMf, a histone from the hyperthermophilic archaeon Methanothermus fervidus binds double-stranded DNA molecules in vitro, forming compact structures that visibly resemble eukaryal nucleosomes. We show here that HMf binding increases the helical periodicity of DNA molecules to almost-equal-to 11 base pairs (bp) per turn and that DNA molecules in these nucleosome-like structures are constrained in positive toroidal supercoils. Based on the mass of HMf needed to cause a change in linking number (DELTA-Lk), the maximum DELTA-Lk introduced into circular DNA molecules of known sizes, and electron microscopy, we estimate that each HMf-DNA structure contains between 90 and 150 bp of DNA wrapped in 1.5 positive toroidal supercoils around a core of four HMf molecules. A model and pathway for the formation of these structures in vitro are presented and the possible role of positive toroidal wrapping of the M. fervidus genome in vivo is discussed.