Reassembly of C- myc and relaxation of C- fos nucleosomes during differentiation of human leukemic (HL-60) cells

Human promyelocytic leukemic (HL-60) cells (1) have amplified c- myc protooncogene sequences which lead to an elevated level of c- myc gene expression (2,3). Induction of HL-60 cells by phorbol esters to undergo monocytic differentiation (4) results in the suppression of c- myc (5), but the activation of c- fos gene transcription (6). Chromatin structures of c- myc and c- fos were compared by measuring their sequences in nucleosome-associated DNA fragments. These nucleosomal particles were released from chromatin by micrococcal nuclease digestion and subsequently analyzed with two dimensional gel electrophoresis. C- myc related sequences were detected in nucleosomal DNA fragments of differentiated cells only, while the c- fos related sequences were found in nucleosomal DNAs of noninduced HL-60 cells. Since the enzyme preferentially digests relaxed DNAs, these results suggest that nucleosomal subunits of c- myc and c- fos chromatin are relaxed during the state of active transcription, and reassembled once their transcription is repressed.