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  • The EMBO Journal (2009) 28, 223 - 233
  • doi:10.1038/emboj.2008.282

Published online: 15 January 2009

Differential targeting of Tetrahymena ORC to ribosomal DNA and non-rDNA replication origins

Taraka R Donti1,a, Shibani Datta1,2,ab, Pamela Y Sandoval2 and Geoffrey M Kapler1,2

  1. Department of Molecular and Cellular Medicine, Texas A&M Health Science Center, College Station, TX, USA
  2. Texas A&M University Interdisciplinary Genetics Program, College Station, TX, USA

Correspondence to:

Geoffrey M Kapler, Department of Molecular and Cellular Medicine, Texas A&M Health Science Center, 454 Reynolds Medical Building, College Station, TX 77843-1114, USA. Tel.: +979 847 8690; Fax: +979 847 9481; E-mail: gkapler@medicine.tamhsc.edu

aThese authors contributed equally to this work

bPresent address: MedImmune Inc., Gaithersburg, MD 20878, USA

Received 8 August 2008; Accepted 2 December 2008

The Tetrahymena thermophila origin recognition complex (ORC) contains an integral RNA subunit, 26T RNA, which confers specificity to the amplified ribosomal DNA (rDNA) origin by base pairing with an essential cis-acting replication determinant—the type I element. Using a plasmid maintenance assay, we identified a 6.7 kb non-rDNA fragment containing two closely associated replicators, ARS1-A (0.8 kb) and ARS1-B (1.2 kb). Both replicators lack type I elements and hence complementarity to 26T RNA, suggesting that ORC is recruited to these sites by an RNA-independent mechanism. Consistent with this prediction, although ORC associated exclusively with origin sequences in the 21 kb rDNA minichromosome, the interaction between ORC and the non-rDNA ARS1 chromosome changed across the cell cycle. In G2 phase, ORC bound to all tested sequences in a 60 kb interval spanning ARS1-A/B. Remarkably, ORC and Mcm6 associated with just the ARS1-A replicator in G1 phase when pre-replicative complexes assemble. We propose that ORC is stochastically deposited onto newly replicated non-rDNA chromosomes and subsequently targeted to preferred initiation sites prior to the next S phase.

  • Keywords:

    • cell cycle,
    • origin recognition complex,
    • replicator,
    • ribonucleoprotein complex,
    • Tetrahymena