Essential protein (PubMed:19171893). Component of the origin recognition complex (ORC) that binds origins of replication. It has a role in both chromosomal replication and mating type transcriptional silencing. Binds to the ARS consensus sequence (ACS) of origins of replication (By similarity). H3K4me3 effector that regulates positively the transcription of a subset of genes (PubMed:19171893). Component of the origin recognition complex (ORC) composed of at least ORC1 (ORC1A or ORC1B), ORC2, ORC3, ORC4, ORC5 and ORC6. ORC is regulated in a cell-cycle and development dependent manner. It is sequentially assembled at the exit from anaphase of mitosis and disassembled as cells enter S phase. Interacts directly with ORC2, ORC3, ORC4 and ORC5 (PubMed:16179646, PubMed:15358564). Binds mostly unmodified histone H3, and, with lower efficiency, H3K4me1 H3K4me2 and H3K4me3 (PubMed:26876097). Follow a cell-cycle regulation with a peak at the G1/S-phase (PubMed:16179646). Mostly expressed in siliques, flowers and flower buds, and, to a lower extent, in roots, leaves and stems (PubMed:16179646, PubMed:15358564). Preferentially expressed in endoreplicating cells, such as cells of the apical hook of dark-grown seedlings. Regulated by E2F (PubMed:16179646). Accumulates rapidly after cell cycle reactivation by sucrose addition following cell cycle arrest mediated by sucrose deprivation (PubMed:16179646, PubMed:15358564). Lethal. Belongs to the ORC1 family.