Construction of nucleosome cores from defined sequence DNA of viral origin.

The de novo construction of defined nucleosomes from two DNA fragments of simian virus SV40 is described. One fragment spans the region containing the origin of replication of the virus from base -16 to base 161, a region which is nucleosome-free during virus replication. The other fragment, of 142 bp (1352 to 1493), is within the region coding for viral proteins VP2 and VP3, and serves for comparison. Both fragments form nucleosomes with similar efficiency when combined with histone cores as well as when exchanged with existing core particles. The DNase I digestion pattern and exonuclease III analysis both indicate that true nucleosome cores are formed, and that a prolonged tail is not protruding from the constructs. The efficient formation of a nucleosome core particle from the origin region of DNA implies that the absence of nucleosomes from this region during viral infection is not prescribed by the specific base sequence of origin DNA, and is therefore likely to be determined by non-histone nuclear factors associated with the SV40 replication process.