Nucleosome "phasing" and cruciform structures in circular supercoiled pBR322 DNA.

Cruciform structures have been detected in pBR322 supercoiled DNA, both in its naked state and when complexed with histone octamer, using S1 endonuclease cleavage and EcoRI restriction. An inspection of the DNA sequence shows that the S1-hypersensitive sites are very near to AT-rich regions of pBR322 genome. A nucleosome "phasing" in these regions, as found on AT-rich regions of SV40 DNA (15), has been shown by restriction enzymes analysis. On the basis of these results it can be proposed that cruciform structures protrude on the nucleosome surface. This model explains the reason why these structures, which need high superhelical density, can exist in supercoiled DNA partially relaxed by nucleosome formation.