Pathways of rDNA copy number homeostasis in Schizosaccharomyces pombe

Fragile sites across the genome pose an increased risk of genetic instability. The rDNA repeats are particularly at risk due to highly repetitive sequences, replication transcription collisions, polar replication fork barriers, and late DNA replication. In this study, we examine mechanisms of rDNA homeostasis in fission yeast. We monitor the effects of an artificially contracted rDNA array, assess the responses to genome-wide alkylation damage, and identify genetic pathways that affect rDNA copy number. We find that a reduced rDNA array leads to a decreased growth rate, smaller cell size, and increased genotoxic sensitivity to alkylation damage from MMS. We also observe that in response to chronic MMS exposure, normal rDNA arrays contract in a time/concentration-dependent manner. We show that rDNA copy number is affected by the fork protection complex (FPC), fork licensing proteins, chromatin modifiers, and DDK kinase. These results confirm that the rDNA repeats are a genome fragile site that is particularly sensitive to perturbations in DNA replication.