Preferential in vitro assembly of nucleosome cores on some AT-rich regions of SV40 DNA.

We have found that nucleosomes reconstituted from histone octamers and SV40 DNA Form I by progressively decreasing the salt concentration from 2 M NaCl are formed preferentially around 0.27, 0.37, 0.50 and 0.85 on SV40 DNA (relative to the EcoRI site). When SV40 DNA Form III is used, the nucleosomes form mainly at 0.28, 0.38, 0.61 and 0.83. These sites are very close to both the sites of RNA chain initiation by calf thymus RNA polymerase B on SV40 DNA Form I (0.25, 0.35, 0.42 and 0.88) and the regions of the supercoiled DNA which are readily denaturable by T4 gene 32 protein (0.25, 0.47 and 0.88), and correspond to AT-rich regions as deduced from the nucleotide sequence of SV40 DNA. The physiologically important region around 0.67 is an unfavourable site for all three types of proteins, and corresponds to a GC-rich region surrounding a 17 base pair AT cluster.