Characterization of the Replication Initiator Orc1/Cdc6 From the Archaeon Picrophilus Torridus

J Bacteriol. 2014 Jan;196(2):276-86. doi: 10.1128/JB.01020-13. Epub 2013 Nov 1.


Eukaryotic DNA replication is preceded by the assembly of prereplication complexes (pre-RCs) at or very near origins in G1 phase, which licenses origin firing in S phase. The archaeal DNA replication machinery broadly resembles the eukaryal apparatus, though simpler in form. The eukaryotic replication initiator origin recognition complex (ORC), which serially recruits Cdc6 and other pre-RC proteins, comprises six components, Orc1-6. In archaea, a single gene encodes a protein similar to both the eukaryotic Cdc6 and the Orc1 subunit of the eukaryotic ORC, with most archaea possessing one to three Orc1/Cdc6 orthologs. Genome sequence analysis of the extreme acidophile Picrophilus torridus revealed a single Orc1/Cdc6 (PtOrc1/Cdc6). Biochemical analyses show MBP-tagged PtOrc1/Cdc6 to preferentially bind ORB (origin recognition box) sequences. The protein hydrolyzes ATP in a DNA-independent manner, though DNA inhibits MBP-PtOrc1/Cdc6-mediated ATP hydrolysis. PtOrc1/Cdc6 exists in stable complex with PCNA in Picrophilus extracts, and MBP-PtOrc1/Cdc6 interacts directly with PCNA through a PIP box near its C terminus. Furthermore, PCNA stimulates MBP-PtOrc1/Cdc6-mediated ATP hydrolysis in a DNA-dependent manner. This is the first study reporting a direct interaction between Orc1/Cdc6 and PCNA in archaea. The bacterial initiator DnaA is converted from an active to an inactive form by ATP hydrolysis, a process greatly facilitated by the bacterial ortholog of PCNA, the β subunit of Pol III. The stimulation of PtOrc1/Cdc6-mediated ATP hydrolysis by PCNA and the conservation of PCNA-interacting protein motifs in several archaeal PCNAs suggest the possibility of a similar mechanism of regulation existing in archaea. This mechanism may involve other yet to be identified archaeal proteins.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Archaeal Proteins / genetics
  • Archaeal Proteins / metabolism
  • Binding Sites
  • Computational Biology
  • DNA Replication*
  • DNA, Archaeal / metabolism
  • Origin Recognition Complex / genetics
  • Origin Recognition Complex / metabolism*
  • Proliferating Cell Nuclear Antigen / metabolism*
  • Protein Binding
  • Protein Multimerization
  • Thermoplasmales / genetics*
  • Thermoplasmales / metabolism*


  • Archaeal Proteins
  • DNA, Archaeal
  • Origin Recognition Complex
  • Proliferating Cell Nuclear Antigen