Abstract
Large doughnut-shaped complexes were formed when histone H1 was mixed with DNA in amounts that extensively neutralized it. The doughnut shape is the most prevalent from observed by electron microscopy for linear double-stranded DNA independent of the molecular weight of the DNA in the range 1.2 x 10(6) to 25 x 10(6) at an H1/DNA input weight ratio of 1.3, at ionic strength 0.17. Doughnuts were not observed for single-stranded DNA-H1 complexes; instead, the complexes were globular. The circumference of the doughnutshaped particles indicates that much of the rigidity of duplex DNA in the complex has remained. Evidently, the condensation of the nucleohistone is constrained by the rigidity of duplex DNA and, under this constraint, surface contact with water is minimized by adopting a doughnut shape. Histone H5 causes a type of DNA condensation similar to that of H1 at comparable charge ratios. Core histones H2A-H2b, H3, and H4 complex with DNA to form globular aggregates of such small diameter that the duplex DNA in them must be much more tightly folded than is the case with the doughnut-shaped complexes. Because these histones are designed to fold DNA into nucleosomes 100 A wide, they must destroy the rigidity of free duplex DNA, perhaps by forming kinks in the chain.
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