Potassium limitation increases MMP and lifespan. (A) MMP ratio as a function of external potassium for WT (blues) and sis2Δ (oranges) in either ammonium sulfate (“NH4,” dark symbols and dark, solid lines) or arginine (“Arg,” light symbols and light, broken lines) as the nitrogen source. The MMP ratio is normalized to the value of WT in standard media (i.e., ammonium sulfate and 7.35 mM potassium). Results are from four experiments with ammonium sulfate and two experiments with arginine as the nitrogen source. n ≥6 replicates for every strain, concentration, and nitrogen source combination, with 10,000 cells analyzed per replicate. (B) Median replicative lifespan as a function of external potassium, estimated from a CPH model (Materials and methods). Data are from three (NH4) or two (Arg) experiments. n ≥1,400 cells for every genotype, nitrogen, and potassium concentration combination. (C and D) MMPs (C) and MMPi (D) signals as a function of external potassium concentration, from the same experiments as A, normalized to WT in ammonium sulfate at 7.35 mM external potassium. All error bars show 95% confidence intervals.