PROXIMAL AND LONG-RANGE ALTERATIONS IN CHROMATIN STRUCTURE SURROUNDING THE CHINESE-HAMSTER DIHYDROFOLATE-REDUCTASE PROMOTER

The chromatin structure of the dihydrofolate reductase (DHFR) gene was examined by DNA/protein cross-linking, chemical DNA methylation, and micrococcal nuclease digestion. The 5' promoter region of the gene dis plays two nucleosome-free zones (-550 to -300 and -150 to +100 bp rela tive to the ATG codon), each of which contains a number of micrococcal nuclease-hypersensitive sites. Regions upstream from the distal hyper sensitive zone (-900 to -550 bp), downstream from the proximal hyperse nsitive zone (+100 to +400 bp), and between these two zones (-300 to - 150 bp) appear either to be more than 80% histone-free or to contain h istones whose globular domains have lost most of their contacts with D NA. Overall, a broad zone extending from -4300 to +4700 bp is altered relative to bulk chromatin, and within this region there are positione d nucleosomes and/or nucleosome-free zones in which the DNA appears to interact with a number of different non-histone proteins. By comparis on, the chromatin in the 3' end of the gene (including the right end o f the 5th intron and the 6th exon) contains randomly positioned nucleo somes, and its structure is intermediate between that of the 5' end of the gene and a downstream matrix attachment region that contains regu larly organized chromatin. A 2.3 kb zone in the central part of the 5t h intron reveals some features similar to the 5' end of the gene, sugg esting a hitherto unrecognized functional role.