Abstract

The incubation of the E. coli DNA binding protein HU with relaxed circular SV40 DNA in the presence of pure nicking-closing enzyme introduces up to 18 negative superhelical turns in the DNA molecules as measured by agarose gel electrophoresis. The maximal density of supercoiling is obtained at a HU-DNA mass ratio of 1. Reconstituted DNA-HU complexes prefixed with glutaraldehyde appear as condensed circular structures having an average of 14 “beads” per circular SV40 DNA molecule, with a “bead” diameter of 180 ± 23 Å. The circular SV40 DNA is condensed by a ratio of 2.0–2.5 relative to naked DNA. This is similar to the ratio (2.4) measured for chromatin formed by reassociation of relaxed SV40 DNA with the four core histones.

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