Identification of a Binding Region for Human Origin Recognition Complex Proteins 1 and 2 That Coincides with an Origin of DNA Replication

We investigated the binding regions of components of the origin recognition complex (ORC) in the human genome. For this purpose, we performed chromatin immunoprecipitation assays with antibodies against human Orc1 and Orc2 proteins. We identified a binding region for human Orc proteins 1 and 2 in a <1-kbp segment between two divergently transcribed human genes. The region is characterized by CpG tracts and a central sequence rich in AT base pairs. Both, Orc1 and Orc2 proteins are found at the intergenic region in the G(1) phase, but S-phase chromatin contains only Orc2 protein, supporting the notion that Orc1p dissociates from its binding site in the S phase. Sequences corresponding to the intergenic region are highly abundant in a fraction of nascent DNA strands, strongly suggesting that this region not only harbors the binding sites for Orc1 protein and Orc2 protein but also serves as an origin of bidirectional DNA replication.