Location of histones on simian virus 40 DNA.

The physical location of histone molecules in a simian virus 40 DNA-histone complex isolated from purified virions was examined using site-specific restriction endonucleases. The complex contains four host histone species but lacks histone F1. Histones prevent complete cleavage of SV40 DNA by two restriction enzymes, HindIII and EcoRI. From the pattern of DNA fragments resulting from cleavage of the histone-DNA complex by the HindIII endonuclease, which makes six breaks on purified SV 40 DNA, we have concluded that histones are randomly arranged on SV40 DNA relative to restriction enzyme cleavage sites. The EcoRI endonuclease, which makes one break in SV40 NDA, was used to determine the degree of physical coverage of the SV 40 DNA molecule by histones. We observed that 80% of the EcoRI sites in the complex are accessible to the enzyme while 20% are "closed." This degree of coverage is consistent with the mass ratio of DNA:histone in the complex as revealed by the buoyant density of the formaldehyde-fixed complex. We conclude that the histones in the complex are located randomly on the SV 40 genome and cover approximatley 20% of the DNA. These results suggest that the histone species F2b, F2al, F2a2, and F3 are bound without regard to nucleotide sequence of SV 40 DNA.