HISTONES H1 AND H5 INTERACT PREFERENTIALLY WITH CROSSOVERS OF DOUBLE-HELICAL DNA

The interaction of the linker histones H1 and H5 from chicken erythroc yte chromatin with pBR322 was studied as a function of the number of s uperhelical turns in circular plasmid molecules. Supercoiled plasmid D NA was relaxed with topoisomerase I so that a population with a narrow distribution of topoisomers, containing from zero to five superhelica l turns, was obtained. None of the topoisomers contained alternative n on-B-DNA structures. Histone-DNA complexes formed at either 25 or 100 mM NaCl final concentration and at histone-DNA molar ratios ranging fr om 10 to 150 were analyzed by agarose gel electrophoresis. The pattern s of disappearance of individual topoisomer bands from the gel were in terpreted as an indication of preference of the linker histones for cr ossovers of double-helical DNA. This preference was observed at both s alt concentrations, being more pronounced under conditions of low ioni c strength. Isolated H5 globular domain also caused selective disappea rance of topoisomers from the gel, but it did so only at very high pep tide-DNA molar ratios. The observed preference of the linker histones for crossovers of double-helical DNA is viewed as a part of the mechan ism involved in the sealing of the two turns of DNA around the histone octamer.