The 5 ' boundary of the human apolipoprotein B chromatin domain in intestinal cells

The 5 ' boundary of the chromosomal domain of the human apolipoprotein B (a poB) gene in intestinal cells has been localized and characterized. It is c omposed of two kinds of boundary elements; the first, functional boundary i s an insulator activity exhibited by a 1.8 kb DNA fragment located between -58 and -56 kb upstream of the human apoB promoter. In this region, an enha ncer-blocking activity has been mapped to a CTCF binding site that is locat ed upstream of; two apoB intestinal enhancers (IEs), the 315 IE and the 485 IE. The CTCF site represents a boundary between two types of chromatin str ucture: an open, DNaseI-sensitive region 3 ' of the CTCF site containing th e intestinal regulatory elements and a closed, DNaseI-resistant region 5 ' of the GTCF site. The 1.8 kb fragment harboring the CTCF site also insulate d mini-white transgenes against position effects in Drosophila melanogaster . The second, structural boundary is represented by a nuclear matrix attach ment region (MAR), situated about 3 kb 5 ' of the CTCF site. This MAR may r epresent the 5 ' anchorage site for a chromosomal loop that functions to br ing the intestinal regulatory elements closer to the apoB promoter.