Figure 4.

Mec1 and Rrd1–Pph3 form a phosphoregulatory circuit that controls telomere healing at DSBs. (A) Model of Rrd1 action, given its role as a PP2A-type phosphatase activator. Rrd1 might activate a phosphatase that antagonizes a phosphorylation event that inhibits telomere healing. (B) Quantitation of spontaneous GCR rates using the Chr V assay. The strains tested were derivatives of hxt13∷URA3 (WT), and, in the same background, pif1Δ, pif1Δ sul2Δ, pif1Δ, pif1Δ rrd1Δ, pif1Δ pph3Δ, pif1Δ ppg1Δ, pif1Δ sit4Δ, and pif1Δ rrd1Δ pph3Δ. The data were obtained following a minimum of two independent fluctuation tests with 11 cultures. The data are presented as the mean ± SEM. (C) Quantitation of telomere addition frequency of wild-type, ppg1Δ, rrd1Δ, pph3Δ, and rrd1Δ pph3Δ derivatives of pif1-m2 TG₅-HO. The data are presented as the mean ± SEM (N ≥ 3). (D) Quantitation of telomere addition frequency of wild-type and mec1Δ derivatives of pif1-m2 sml1Δ TG₀-HO (left panel), and wild-type, tel1Δ, and rad9Δ derivatives of pif1-m2 TG₀-HO (right panel). SML1 was deleted to maintain the viability of mec1Δ. The data are presented as the mean ± SEM (N ≥ 3). (E) Quantitation of telomere addition frequency of wild-type and mec1Δ derivatives of pif1-m2 sml1Δ TG₅-HO. The telomere addition frequency of mec1Δ derivatives containing plasmids that expressed either MEC1 or a mec1 kinase-dead allele (mec1-kd) is also shown. The data are presented as the mean ± SEM (N ≥ 3). (F) Quantitation of telomere addition frequency of wild-type, mec1Δ, rrd1Δ, and mec1Δ rrd1Δ derivatives of pif1-m2 sml1Δ TG₅-HO. The data are presented as the mean ± SEM (N ≥ 3). (G) Quantitation of telomere addition frequency of wild-type, mec1Δ, rrd1Δ, sml1Δ, and mec1Δ sml1Δ derivatives of pif1-m2 TG₁₁-HO. The data are presented as the mean ± SEM (N ≥ 3).