In the yeast Saccharomyces cerevisiae, chromosomes terminate with a repetitive sequence [poly(TG 1-3 )] 350 to 500 bp in length. Strains with a mutation of TEL1, a homolog of the human gene (ATM) mutated in patients with ataxia telangiectasia, have short but stable telomeric repeats. Mutations of TLC1 (encoding the RNA subunit of telomerase) result in strains that have continually shortening telomeres and a gradual loss of cell viability; survivors of senescence arise as a consequence of a Rad52p-dependent recombination events that amplify telomeric and subtelomeric repeats. We show that a mutation in MEC1 (a gene related in sequence to TEL1 and ATM) reduces telomere length and that tel1 mec1 double mutant strains have a senescent phenotype similar to that found in tlc1 strains. As observed in tlc1 strains, survivors of senescence in the tel1 mec1 strains occur by a Rad52p-dependent amplification of telomeric and subtelomeric repeats. In addition, we find that strains with both tel1 and tlc1 mutations have a delayed loss of cell viability compared to strains with the single tlc1 mutation. This result argues that the role of Tel1p in telomere maintenance is not solely a direct activation of telomerase.Most eukaryotic chromosomes end with simple repetitive DNA sequences (7). In the yeast Saccharomyces cerevisiae, wild-type strains have poly(TG 1-3 ) tracts 350 to 500 bp in length (31,42,43). Within a yeast cell population, telomeric tracts, even for a single chromosome, range in size by about Ϯ 50 bp (43). This variability in telomere length, as well as the identification of mutants with telomeric tracts that are longer or shorter than those in wild-type strains, suggests that telomere length is likely to reflect a balance between mechanisms that extend or contract these terminal repeats.In many eukaryotes, including yeast, the most important mechanism for extending telomeric repeats is telomerase (7,45). This RNA-protein enzyme complex extends the G-rich strand of the telomere 5Ј to 3Ј, using telomeric repeats encoded within the RNA component of the enzyme as a template. The extended G-rich strand is thought to be copied 5Ј to 3Ј by conventional DNA polymerases to yield the complementary C-rich strand (7). In yeast, the RNA component of telomerase is encoded by TLC1 (33), and the protein component with reverse transcriptase activity is encoded by EST2 (16). In strains with mutations in either of these genes, telomeric tracts shorten with each cell division, resulting in gradual loss of cell viability (13,33). This same senescent phenotype is observed in strains with mutations in several other EST (ever shorter telomeres) genes, including EST1, EST3, and CDC13/EST4 (13,19). Since mutations in all EST genes have the same phenotype and since double-mutant est strains have the same phenotype as the single mutants, it has been suggested that all Est proteins function in the same pathway of telomere maintenance (13, 23, 40), although not necessarily as components of telomerase (15).Although yeast strains with a mutatio...

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Fungal infections arise frequently in immunocompromised patients, and sterol synthesis is a primary pathway targeted by antifungal drugs. In particular, the P450 protein Erg11/Cyp51 catalyzes a critical step in ergosterol synthesis, and the azole class of antifungal drugs inhibits Erg11. Dap1 is a heme-binding protein related to cytochrome b 5 that activates Erg11, so that cells lacking Dap1 accumulate the Erg11 substrate and are hypersensitive to Erg11 inhibitors. Heme binding by Dap1 is crucial for its function, and point mutants in its heme-binding domain render Dap1 inactive for sterol biosynthesis and DNA damage resistance. Like Dap1, the human homologue, PGRMC1/Hpr6, also regulates sterol synthesis and DNA damage resistance. In the present study, we demonstrate that the Dap1 heme-1 domain is required for growth under conditions of low iron availability. Loss of Dap1 is suppressed by elevated levels of Erg11 but not by increased heme biosynthesis. Dap1 localizes to punctate cytoplasmic structures that co-fractionate with endosomes, and Dap1 contributes to the integrity of the vacuole. The results suggest that Saccharomyces cerevisiae Dap1 stimulates a P450-catalyzed step in sterol synthesis via a distinct localization from its homologues in Schizosaccharomyces pombe and mammals and that this function regulates iron metabolism.