Specific origin selection and excess functional MCM2-7 loading in ORC-deficient cells

Origin Recognition Complex (ORC) is composed of six subunits, ORC1-6, and the entire complex loads excess MCM2-7 on chromosomes to promote initiation of DNA replication in organisms ranging from S. cerevisiae to humans. ORC is also believed to be important for origin specification. Mapping of origins in the cancer cell lines engineered to delete three of the subunits, ORC1, ORC2 or ORC5 shows that origin specification continues at mostly the same sites in the absence of ORC. The few hundred origins that were up-regulated in the absence of ORC suggest that GC/TA skewness and simple repeat sequences facilitate origin selection in the absence of the six-subunit ORC. In the absence of ORC, MCM2-7 association with chromatin continues to be sufficiently in excess to license reserve origins, used under replication stress, and to permit re-replication, which requires repeated loading of MCM2-7 at origins in the same cell-cycle. Thus, origin specification and excess MCM2-7 loading on origins can be executed in mammalian cancer cells in the absence of the six-subunit ORC. Overall design: SNS-seq for HCT116 WT and ORC1, ORC2 and ORC5 knockout cells