ONGOING ACTIVITY OF RNA-POLYMERASE-II PRECLUDES CHROMATIN COLLAPSE AND DNA FRAGMENTATION IN CHINESE-HAMSTER OVARY CELLS

The role of ongoing RNA synthesis in chromatin organization in Chinese hamster ovary cells was examined upon exposure to the transcription i nhibitor alpha-amanitin. Treatment with alpha-amanitin led to pleomorp hic nuclei with chromatin heavily condensed and with the remaining rib onucleoprotein aggregated in large compact granular masses around the margins of the nuclear periphery. Concommitant with the changes in nuc lei morphology transient focal dilatation of the rough endoplasmic ret iculum was observed while other cytoplasmic organelles appeared struct urally unaffected. The morphological changes occurred after complete i nhibition of RNA polymerase II mediated transcription. The molecular i ntegrity of isolated DNA was monitored in parallel with the structural analysis. Fragmentation of cellular DNA occured in a time-dependent f ashion and well after the complete inhibition of RNA synthesis. Charac teristic oligonucleosome-sized DNA fragments of about 187 base pairs i n length was produced in a cotemporal time-dependent fashion. Our find ings indicate that ongoing transcription and the structural state of c hromatin are very closely integrated, and provide further evidence tha t RNA is a structural component of the nuclear matrix, which in turn i s involved in keeping chromatin physically dispersed and decondensed. (C) 1996 Academic Press, Inc.