Replication Stress Leads to Genome Instabilities inArabidopsisDNA Polymerase Δ Mutants

Abstract

Impeded DNA replication or a deficiency of its control may critically threaten the genetic information of cells, possibly resulting in genome alterations, such as gross chromosomal translocations, microsatellite instabilities, or increased rates of homologous recombination (HR). We examined an Arabidopsis thaliana line derived from a forward genetic screen, which exhibits an elevated frequency of somatic HR. These HR events originate from replication stress in endoreduplicating cells caused by reduced expression of the gene coding for the catalytic subunit of the DNA polymerase Δ (POLΔ1). The analysis of recombination types induced by diverse alleles of polΔ1 and by replication inhibitors allows the conclusion that two not mutually exclusive mechanisms lead to the generation of recombinogenic breaks at replication forks. In plants with weak polΔ1 alleles, we observe genome instabilities predominantly at sites with inverted repeats, suggesting the formation and processing of aberrant secondary DNA structures as a result of the accumulation of unreplicated DNA. Stalled and collapsed replication forks account for the more drastic enhancement of HR in plants with strong polΔ1 mutant alleles. Our data suggest that efficient progression of DNA replication, foremost on the lagging strand, relies on the physiological level of the polymerase Δ complex and that even a minor disturbance of the replication process critically threatens genomic integrity of Arabidopsis cells.