INTERACTION BETWEEN THE ORIGIN RECOGNITION COMPLEX AND THE REPLICATION LICENSING SYSTEM IN XENOPUS

The origin recognition complex (ORC) binds to origins of replication i n budding yeast. We have cloned a Xenopus homolog of the largest ORC p olypeptide (XORC1). Immunodepletion of XOrc1 from Xenopus egg extracts blocks the initiation of DNA replication. We have purified Xenopus OR C, consisting of a protein complex similar to yeast ORC. In Xenopus eg g extracts, ORC associates with chromatin throughout G1 and S phases. RLF-M, a component of the replication licensing system, also associate s with chromatin early in the cell cycle but dissociates during S phas e. We show that the assembly of RLF-M onto chromatin is dependent on t he presence of chromatin-bound ORC, leading to sequential assembly of initiation proteins onto replication origins during the cell cycle.