Position-dependent instability of ribosomal DNA repeats (736.1)

Repetitive DNA arrays are ubiquitous in eukaryotic genomes and represent a special challenge for genome integrity because of the risk of non-allelic recombination. We used unique sequence tags marking individual repeats in the tandem ribosomal DNA (rDNA) array of budding yeast to determine the likelihood of non-allelic recombination throughout the array. We found that rDNA repeat stability during meiotic recombination is strongly dependent on relative repeat position, with repeats near the rDNA border being more susceptible to meiotic DNA double-strand breaks (DSBs) and thus non-allelic recombination. The DSB position effect is promoted by rDNA heterochromatin and suppressed by the origin recognition complex component Orc1. Border-dependent position effects in the rDNA also occur at the level of reporter gene expression in vegetative cells, suggesting that such effects are a reflection of a general feature of rDNA structure. Our results indicate that, despite their identity at the sequence level, rDNA rep...