Role for a Xenopus Orc2-related protein in controlling DNA replication

THE six-subunit origin recognition complex (ORC) is essential for the initiation of DNA replication at specific origins in the budding yeast Saccharomyces cerevisiae(1-9). An important issue is whether DNA replication in higher eukaryotes, in which the characteristics of replication origins are poorly defined(10), occurs by an ORC-dependent mechanism, We have identified a Xenopus laevis Orc2-related protein (XORC2) by its ability to rescue a mitotic-catastrophe mutant of the fission yeast Schizosaccharomyces pombe, We show that immunodepletion of XORC2 from Xenopus egg extracts(11-13) abolishes the replication of chromosomal DNA but not elongation synthesis on a single-stranded DNA template, Indirect immunofluorescence indicates that XORC2 binds to chromatin well before the commencement of DNA synthesis, and even under conditions that prevent the association of replication licensing factor(s) with the DNA. These findings suggest that Orc2 plays an important role at an early step of chromosomal replication in animal cells.