CHANGES IN CHROMATIN STRUCTURE SUPPORT CONSTITUTIVE AND DEVELOPMENTALLY-REGULATED TRANSCRIPTION OF THE BONE-SPECIFIC OSTEOCALCIN GENE IN OSTEOBLASTIC CELLS

Transcription of the osteocalcin gene, which encodes a 10 kDa bone-spe cific protein, is controlled by modularly organized basal regulatory s equences and hormone-responsive enhancer elements, We have previously shown that in the ROS 17/2.8 rat osteosarcoma cell line, which continu ously expresses the osteocalcin gene, key regulatory elements reside i n two DNase I hypersensitive sites that are functionally correlated wi th transcriptional activity. We now report that a specific nucleosomal organization supports this constitutive expression in ROS 17/2.8 cell s, and that chromatin remodeling directly correlates with the developm entally regulated transcriptional activation of the osteocalcin gene d uring differentiation of normal diploid rat osteoblasts. By combining DNase I, micrococcal nuclease, and specific restriction endonuclease d igestion analysis, we observed that the presence of DNase I hypersensi tive sites (-170 to -70 and -600 to -400) and a selective nucleosome p ositioning over the OC gene promoter are directly associated with deve lopmental stage-specific transcriptional activation in bone-derived ce lls.