Proteins of the origin recognition complex (ORC) and DNA topoisomerases on mammalian chromatin

The process of DNA replication requires the separation of complementary DNA strands. In this process, the unwinding of circularly closed or long DNA duplices leads to torsional tensions which must be released by topoisomerases. So topoisomerases play an important role in DNA replication. In order to provide more information about topoisomerases in the initiation of mammalian replication, we investigated whether topoisomerases occur close to ORC in the chromatin of cultured human HeLa cells.

We have used different cell fractionation procedures, namely salt and nuclease treatment of isolated nuclei as well as formaldehyde-mediated cross-linking of chromatin, to investigate the distribution of topoisomerases and proteins of the origin recognition complex (ORC) in the chromatin of human HeLa cells. First we obtained no evidence for a physical interaction of either topoisomerase I or topoisomerase II with ORC. Then we found, however, that (Orc1-5) and topo II occurred together on chromatin fragments of 600 and more bp lengths. At last we showed that both topo II and Orc2 protein are enriched near the origin at the human MCM4 gene, and at least some of the topo II at the origin is active in proliferating HeLa cells. So taken together, topoisomerase II, but not topoisomerase I, is located close to ORC on chromatin.

Topoisomerase II is more highly expressed than ORC proteins in mammalian cells, so only a small fraction of total chromatin-bound topoisomerase II was found in the vicinity of ORC. The precise position of topo II relative to ORC may differ among origins.