Changes of telomere length cause reciprocal changes in the lifespan of mother cells in Saccharomyces cerevisiae

Abstract

Budding yeast cells divide asymmetrically, giving rise to a mother and its daughter. Mother cells have a limited division potential, called their lifespan, which ends in proliferation-arrest and lysis. In this report we mutate telomerase in Saccharomyces cerevisiae to shorten telomeres and show that, rather than shortening lifespan, this leads to a significant extension in lifespan. This extension requires the product of the SIR3 gene, an essential component of the silencing machinery which binds to telomeres. In contrast, longer telomeres in a genotypically wild-type strain lead to a decrease in lifespan. These findings suggest that the length of telomeres dictates the lifespan by regulating the amount of the silencing machinery available to nontelomeric locations in the yeast genome.

Figure 1

Overexpression of truncated TLC1 results in loss of telomeric silencing, shortened telomeres, and lengthened lifespan. (A) Genomic DNA isolated from the indicated strains was digested with XhoI and electrophoresed on a 1.5% agarose gel and probed with a telomere probe. The broad band at the bottom of the gel migrating at about 1 kb consists of yeast telomeres. (B) Mortality curves are shown for the wild-type and Δsir3 strains containing either an empty vector or a vector overexpressing the truncated form of TLC1. The mean lifespans were 25.4, 20.3, 29.2, and 20.6 generations for the W303 (⊡), W303 Δsir3 (♦), W303 ADH-tTLC1 (▪), and W303 Δsir3 ADH-tTLC1 (⋄) strains, respectively. Corresponding sample sizes were 45, 45, 45, and 48 cells, respectively.

Figure 2

Deletion of RIF1 decreases lifespan. Mortality curves are shown for the wild-type (⊡), Δrif1 (□), and Δsir3 (⋄) strains in the W303 strain background (20). Mean lifespans were 27.3, 19.1, and 20.0 generations, respectively. Corresponding sample sizes were 52, 31, and 34 cells.

Figure 3

Mutations in SIR4 that disrupt recruitment of the SIR proteins to telomeres suppress the short-lifespan phenotype of the Δrif1 mutant. (A) Mortality curves are shown for the wild type (⊡), the Δrif1 (♦) strain, and the Δrif1 strain containing the anti-SIR4 construct (▪). Controls contained an empty vector. Mean lifespans were 27.1, 20.2, and 23.4 generations, respectively. Corresponding sample sizes were 67, 46, and 28 cells. (B) Mortality curves are shown for the wild type (⊡), the Δrif1 (♦) strain, and the Δrif1 strain carrying an integrated SIR4-42 allele (▪). Mean lifespans were 24.5, 16.8, and 19.3 generations, respectively. Corresponding sample sizes were 24, 45, and 48 cells. All mortality curves were obtained from two independent experiments, and lifespan analysis was done as described (22).

Figure 4

Lifespan is inversely correlated with telomere length in wild-type yeast cells. (A) Wild-type yeast strains which have different telomere lengths were isolated as described in the text. Genomic DNA isolated from the indicated strains was digested with XhoI, electrophoresed on a 1.5% agarose gel, and probed with a telomere probe. The broad band at the bottom of each lane in the gel consists of yeast telomeres, which are heterogeneous in length. (B) Mortality curves are shown for the wild-type (□), long telomeres–early passage (○), and long telomeres–later passage (⋄) strains in the W303 strain background (20). Mean lifespans were 24.9, 21.6, and 22.8 generations, respectively. Corresponding sample sizes were 21, 44, and 23 cells.

Figure 5

Telomeres regulate lifespan by modulating genomic silencing. In this model, we propose that shortening of telomeres (arrowheads) results in less recruitment of the SIR silencing machinery (ovals) to telomeres and a concomitant redistribution of this machinery to nontelomeric sites. This redistribution causes a delay in senescence. Conversely, lengthening of telomeres results in greater recruitment of the silencing machinery and an acceleration of senescence.