A number of DNaseI-hypersensitive (DH) sites have been mapped within a 
regulatory region situated upstream of the human apolipoprotein B (apoB) 
promoter (-5262 to -899) that is required for high level expression of human 
apoB transgenes in the livers of mice. These DH sites were observed in nuclei 
from transcriptionally active liver-derived HepG2 cells, but were absent from 
transcriptionally inactive HeLa cell nuclei. Several nuclear protein binding 
sites were detected in the DNaseI-hypersensitive region by DNaseI footprinting 
with HepG2 nuclear extracts, representing putative binding sites for the 
liver-specific activators. The locations of binding sites for these 
transcription factors were revealed via computer analysis of the DNA sequence of 
this region against a transcription factor database. Many micrococcal nuclease 
hypersensitive (MH) sites were also observed in nuclei from HepG2 cells but not 
in HeLa cell nuclei, implying that in hepatic cells, nucleosomes are either 
absent or have been displaced from this region by the liver-specific 
transcriptional activators, as inferred by the correspondence between the DH 
sites, the MH sites and the footprints.