Dynamics of DNA replication initiation complex in fission yeast during cell cycle

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The origin recognition complex (ORC) has been identified as a protein complex of six subunits which binds to an replication origin in S. cerevisiae. The role of ORC is considered as a landing pad for replication factors. The subsequent studies suggest the involvement of ORC in several processes other than DNA replication including silencing, checkpoint control, and mitosis. However, it had been unclear to what extent ORC is involved in the regulation of these processes. We characterized a fifth subunit of ORC in S. pombe, Orp5 (ORC related protein 5). The orp5ィイD1+ィエD1 gene was essential for cell viability, and its depletion resulted in incomplete S-phase and inability to enter mitosis. Further analysis of three different alleles of orp5 temperature sensitive mutants elucidated the multiple functions of Orp5 which were separable : the initiation of DNA replication, S/M-checkpoint, S-phase progression, S-phase checkpoint, and mitosis. The orp5-H19 mutant was defective in DNA replication initiation and checkpoint control which prevent mitosis until S-phase is complete. The other mutant, orp5-K37 appeared to exit G1-phase, but the S-phase was stalled. The orp5-H37 has a deficiency entering M-phase while DNA synthesis itself is complete. In addition, the orp5-H37 was defective in DNA-damage checkpoint in S-phase which are regulated by rad3-cds1 pathway. Taken together, we propose that ORC is a chromatin component which constitutes a base for these events and manage cells to proceed the cell cycle properly.