Chromosome integrity inSaccharomyces cerevisiae: the interplay of DNA replication initiation factors, elongation factors, and origins

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Abstract
The integrity of chromosomes during cell division is ensured by bothtrans-acting factors andcis-acting chromosomal sites. Failure of either these chromosome integrity determinants (CIDs) can cause chromosomes to be broken and subsequently misrepaired to form gross chromosomal rearrangements (GCRs). We developed a simple and rapid assay for GCRs, exploiting yeast artificial chromosomes (YACs) inSaccharomyces cerevisiae. We used this assay to screen a genome-wide pool of mutants for elevated rates of GCR. The analyses of these mutants define new CIDs (Orc3p, Orc5p, and Ycs4p) and new pathways required for chromosome integrity in DNA replication elongation (Dpb11p), DNA replication initiation (Orc3p and Orc5p), and mitotic condensation (Ycs4p). We show that the chromosome integrity function of Orc5p is associated with its ATP-binding motif and is distinct from its function in controlling the efficiency of initiation of DNA replication. Finally, we used our YAC assay to assess the interplay oftransandcisfactors in chromosome integrity. Increasing the number of origins on a YAC suppresses GCR formation in ourdpb11mutant but enhances it in ourorcmutants. This result provides potential insights into the counterbalancing selective pressures necessary for the evolution of origin density on chromosomes.