Subtelomeric duplications in Saccharomyces cerevisiae chromosomes III and XI: topology, arrangements, corrections of sequence and strain-specific polymorphism

Gex1 is a yeast glutathione exchanger that interferes with pH and redox homeostasis

In the yeast Saccharomyces cerevisiae, glutathione plays a major role in heavy metal detoxification and protection of cells against oxidative stress. We show that Gex1 is a new glutathione exchanger. Gex1 and its paralogue Gex2 belong to the major facilitator superfamily of transporters and display similarities to the Aft1-regulon family of siderophore transporters. Gex1 was found mostly at the vacuolar membrane and, to a lesser extent, at the plasma membrane. Gex1 expression was induced under conditions of iron depletion and was principally dependent on the iron-responsive transcription factor Aft2. However, a gex1Δ gex2Δ strain displayed no defect in known siderophore uptake. The deletion mutant accumulated intracellular glutathione, and cells overproducing Gex1 had low intracellular glutathione contents, with glutathione excreted into the extracellular medium. Furthermore, the strain overproducing Gex1 induced acidification of the cytosol, confirming the involvement of Gex1 in proton transport as a probable glutathione/proton antiporter. Finally, the imbalance of pH and glutathione homeostasis in the gex1Δ gex2Δ and Gex1-overproducing strains led to modulations of the cAMP/protein kinase A and protein kinase C1 mitogen-activated protein kinase signaling pathways.

The Saccharomyces retrotransposon Ty5 influences the organization of chromosome ends

Retrotransposons are ubiquitous components of eukaryotic genomes suggesting that they have played a significant role in genome organization. In Saccharomyces cerevisiae, eight of 10 endogenous insertions of the Ty5 retrotransposon family are located within 15 kb of chromosome ends, and two are located near the subtelomeric HMR locus. This genomic organization is the consequence of targeted transposition, as 14 of 15 newly transposed Ty5 elements map to telomeric regions on 10 different chromosomes. Nine of these insertions are within 0.8 kb and three are within 1.5 kb of the autonomously replicating consensus sequence in the subtelomeric X repeat. This suggests that the X repeat plays an important role in directing Ty5 integration. Analysis of endogenous insertions from S.cerevisiae and its close relative S.paradoxus revealed that only one of 12 insertions has target site duplications, indicating that recombination occurs between elements. This is further supported by the observation that Ty5 insertions mark boundaries of sequence duplications and rearrangements in these species. These data suggest that transposable elements like Ty5 can shape the organization of chromosome ends through both transposition and recombination.