We offer two simple models to explain the requirement for FOB1 in the age-associated increase in LOH at MET15. Fob1 activity may be required to generate an aging factor or process that leads to increased LOH. Alternatively, an aging process may occur independent of Fob1, while Fob1 remains necessary for this process to manifest as LOH events in old cells. To distinguish between these modes of action, we generated a diploid MEP strain in which FOB1 is expressed from a tetracycline-repressible promoter [27]. When the TET-FOB1 strain is aged over a 70-hour time course in the absence of repressor, cells exhibit a normal increase in LOH (Figure 4; Fob1 ON). In contrast, when the TET-FOB1 strain is aged over a 70-hour time course in the presence of doxycycline (a tetracycline analog), it behaves as a fob1 null allele and age-associated LOH is suppressed (Figure 4; Fob1 Off). Critically, after the TET-FOB1 strain is aged over a 65-hour time course in the presence of doxycycline, removal of the repressor results in a rapid ∼5-fold increase in LOH levels within 5 hours
