The effect of disrupting sister chromatid cohesion was examined by deleting the genes encoding the cohibin complex proteins Csm1 and Lrs4. The cohibins physically interact with both Fob1 and cohesins, anchoring cohesin rings within the rDNA array and inhibiting both unequal sister chromatid exchange (USCE) in haploid cells and homologous recombination in diploids [17], [31]. Consistent with these reports, we found that csm1Δ or lrs4Δ alleles resulted in high rates of MET15 LOH in young cells (Figure 5). But importantly, when csm1Δ or lrs4Δ were combined with fob1Δ, rDNA recombination rates in young cells remained high (Figure 5). Thus, loss of cohesion leads to Fob1-independent rDNA recombination, which strongly suggests that this is unlikely to be the mechanism underlying the increased rDNA recombination observed in aging cells.
