A portion of the nucleoprotein containing viral DNA extracted from cells infected by simian virus (SV40) is preferentially cleaved by endonucleases in a region of the genome encompassing the origin of replication and early and late promoters. To explore this nuclease-sensitive structure, we cleaved SV40 chromatin molecules with restriction enzymes and digested the exposed termini with nuclease Bal3l. Digestion proceeded only a short distance in the late direction from the MspI site, but some molecules were degraded 400 to 500 base pairs in the early direction. By comparison, BglI-cleaved chromatin was digested for only a short distance in the early direction, but some molecules were degraded 400 to 450 base pairs in the late direction. These barriers to Bal3l digestion (bracketing the BglI and the MspI sites) define the borders of the same open region in SV40 chromatin that is preferentially digested by DNase I and other endonucleases. In a portion of the SV40 chromatin, Bal3l could not digest through the nuclease-sensitive region and reached barriers after digesting only 50 to 100 base pairs from one end or the other. Chromatin molecules that contain barriers in the BglI to MspI region are physically distinct from molecules that are open in this region as evidenced by partial separation of the two populations on sucrose density gradients.Viral nucleoprotein complex extracted from cells infected by simian virus 40 (SV40) resembles cellular chromatin (1,10,15,27). A short segment of the viral genome (ca. 400 base pairs [bp] extending from the SV40 origin of replication in the late direction) is hypersensitive to endonuclease cleavage (33,(38)(39)(40) and appears, in the electron microscope, as a region which is free of nucleosomes (19,32). Recently, more detailed studies have shown that, within the nuclease-sensitive site, there are subregions with differential sensitivity to DNase 1 (7,8,31,32). Presumably, these features reflect the structural details of the nucleoprotein within this region.Nuclease-hypersensitive sites usually occur over regions which contain genetic signals. Such a chromatin structure over a eucaryotic promoter may be a necessary (but not sufficient) condition for transcription from that promoter (6,12,43). In addition, hypersensitive sites have been reported which are probably not related to promoter function (3,22,28). SV40-infected cells provide a good opportunity to investigate the details of the nuclease-sensitive chromatin structure, since SV40 chromatin is present in high concentration by comparison with other genes and can easily be separated from bulk cellular chromatin.We have developed a new approach for probing nucleoprotein structure which should be applicable to the study of chromatin structure in other minichromosomes. SV40 is cleaved with a restriction enzyme which cuts at a unique position, and the exposed termini are digested with Bal3l, an exonuclease which can degrade from either 3' or 5' ends.
MATERIALS AND METHODSGrowth of cells and virus infections. African green monk...