The Origin Recognition Complex (ORC) is the first to access DNA fro replication initiation in eukaryotes. The ORC is composed of six highly conserved subunits in all eukaryotic lineages. However, in contrast to metazoans, Arabidopsis thaliana has acquired two ORC1 genes that encode proteins possessing around 92% amino acid similarity.
In a study published in the journal Nature Communication, scientists from the CBM Severo Ochoa and the Institute of Molecular Biology of Plants (IBMP) investigated the cellular localization and the mutant phenotypes of both Arabidopsis ORC1 proteins to define their respective functional roles. ORC1b is cell cycle regulated and involved in DNA replication for efficient entry in S-phase upon its proteasome degradation. On the contrary, ORC1a is restricted to endoreplicating cells and involved in heterochromatin maintenance. This division of labor that did not occur in animals highlights the essential connection between DNA replication and heterochromatin maintenance to preserve genome stability.