Originally published as Genetics Published Articles Ahead of Print on August 24, 2007.

Genetics, Vol. 177, 1445-1458, November 2007, Copyright © 2007
doi:10.1534/genetics.107.074690

Identification of Mutations That Decrease the Stability of a Fragment of Saccharomyces cerevisiae Chromosome III Lacking Efficient Replicators

James F. Theis^*, Ann Dershowitz^*, Carmela Irene^*, Clelia Maciariello, Michael L. Tobin^*, Giordano Liberi, Sahba Tabrizifard^*,1, Malgorzata Korus^*, Lucia Fabiani and Carol S. Newlon^*,2

^* Department of Microbiology and Molecular Genetics, UMDNJ-New Jersey Medical School, Newark, New Jersey 07103, Dipartimento di Biologia Cellulare e dello Sviluppo, Università "La Sapienza," 00185 Rome, Italy and FIRC Institute of Molecular Oncology Foundation and Department of Biomedical Sciences and Biotechnology, Università degli Studi di Milano, 20139 Milan, Italy

² Corresponding author: Department of Microbiology and Molecular Genetics, UMDNJ-New Jersey Medical School, 225 Warren St., Newark, NJ 07103.
E-mail: newlon{at}umdnj.edu

Eukaryotic chromosomes are duplicated during S phase and transmitted to progeny during mitosis with high fidelity. Chromosome duplication is controlled at the level of replication initiation, which occurs at cis-acting replicator sequences that are spaced at intervals of 40 kb along the chromosomes of the budding yeast Saccharomyces cerevisiae. Surprisingly, we found that derivatives of yeast chromosome III that lack known replicators were replicated and segregated properly in at least 96% of cell divisions. To gain insight into the mechanisms that maintain these "originless" chromosome fragments, we screened for mutants defective in the maintenance of an "originless" chromosome fragment, but proficient in the maintenance of the same fragment that carries its normal complement of replicators (originless fragment maintenance mutants, or ofm). We show that three of these Ofm mutations appear to disrupt different processes involved in chromosome transmission. The OFM1-1 mutant seems to disrupt an alternative initiation mechanism, and the ofm6 mutant appears to be defective in replication fork progression. ofm14 is an allele of RAD9, which is required for the activation of the DNA damage checkpoint, suggesting that this checkpoint plays a key role in the maintenance of the "originless" fragment.

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