Lastly, we examined LOH rates in aging sir2Δ cells to determine if there is an age-associated increase in the absence of Sir2. Consistent with our earlier results [11], there was a very high level of LOH in young cells. Nevertheless, after 70 hours of replicative aging, LOH rates at MET15 increased significantly (p = 0.0003, Fisher's exact test based on contingency tables; Figure 7C). It is worth noting that sir2Δ cells have a short RLS [32]; thus at 70 hours only the longest-lived 1% of the total population are represented. When taken together, these data - the lack of correlation between Sir2 protein levels and rDNA recombination rates in aging cells, the difference in reciprocal/non-reciprocal LOH events between aging wild type and sir2Δ cells, and the Sir2-independent increase in LOH in aging cells - indicate that declining Sir2 levels are insufficient to explain the age-associated rDNA recombination phenotype.
