The appearance of DNase I hypersensitive sites at the 5' end of the late SV40 genes is correlated with the transcriptional switch.

DNase I digestion of the SV40 nuclear chromosome late in infection reveals three hypersensitive sites on the late side of the Bg1 I site. Two of these sites at bp 370, 270 correspond to the 5' side of the late transcripts while the third at bp 190, to a region that is required for early transcription. Early in infection, as well as in an SV40 transformed cell line and a T-Ag negative revertant (deleted in the coding region for T-Ag) only one of these sites is present - the one associated with early transcription. Thus, the positions of these major hypersensitive sites are related to the differential expression of the early and late genes. The presence of the characteristic hypersensitive site corresponding to "early" region expression in the revertant, where large T antigen is not synthesized, but where the early "promotor" is intact, indicates that large T antigen is not responsible for this particular hypersensitive site. Additional minor specific DNase I cuts were found on the early genes, at early times only, at 300, 550, 850 bp from Bg1 I site. In the transformed cell line, one of these minor cuts is found about 350-400 bp from the Bg1 I site and in the revertant, where this region is deleted, a new site is created at 100 bp.