CONCENTRATION OF PROTOONCOGENES IN HEPATOCYTE NUCLEI

Sequential activation of nuclear protooncogenes c-fos, c-jun, and c-my c initiated by cycloheximide in rat hepatocytes were shown to be coupl ed with sequential changes in chromatin structure: decreasing suscepti bility of the bulk of chromatin to endogenous Ca/Mg-dependent DNase an d redistribution of chromatin solubility in low- and high-ionic-streng th media. In the nuclei of differentiated hepatocytes, inactive c-myc and c-fos concentrated in the DNase-resistant chromatin fraction (5-20 % of total). When protein synthesis was suppressed for 2-3 h, the bulk of chromatin became DNase-resistant and insoluble at low ionic streng th. In contrast, the chromatin containing c-myc and c-fos proved sensi tive to the endogenous DNase, whereby the oncogenes could be released into solution with appreciable retention of structure and functional a ctivity. This served as a basis for developing the method for concentr ating and isolating small chromatin fractions enriched in inactive or active c-myc and c-fos.