Yeast cells contain a family of three monothiol glutaredoxins: Grx3, 4, and 5. Absence of Grx5 leads to constitutive oxidative damage, exacerbating that caused by external oxidants. Phenotypic defects associated with the absence of Grx5 are suppressed by overexpression of SSQ1 and ISA2, two genes involved in the synthesis and assembly of iron/sulfur clusters into proteins. Grx5 localizes at the mitochondrial matrix, like other proteins involved in the synthesis of these clusters, and the mature form lacks the first 29 amino acids of the translation product. Absence of Grx5 causes: 1) iron accumulation in the cell, which in turn could promote oxidative damage, and 2) inactivation of enzymes requiring iron/sulfur clusters for their activity. Reduction of iron levels in grx5 null mutants does not restore the activity of iron/sulfur enzymes, and cell growth defects are not suppressed in anaerobiosis or in the presence of disulfide reductants. Hence, Grx5 forms part of the mitochondrial machinery involved in the synthesis and assembly of iron/sulfur centers.

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We have developed an activator/repressor expression system for budding yeast in which tetracyclines control in opposite ways the ability of tetR-based activator and repressor molecules to bind tetO promoters. This combination allows tight expression of tetO- driven genes, both in a direct (tetracycline-repressible) and reverse (tetracycline-inducible) dual system. Ssn6 and Tup1, that are components of a general repressor complex in yeast, have been tested for their repressing properties in the dual system, using lacZ and CLN2 as reporter genes. Ssn6 gives better results and allows complete switching-off of the regulated genes, although increasing the levels of the Tup1-based repressor by expressing it from a stronger promoter improves repressing efficiency of the latter. Effector-mediated shifts between expression and non-expression conditions are rapid. The dual system here described may be useful for the functional analysis of essential genes whose conditional expression can be tightly controlled by tetracyclines.

Grx3 and Grx4, two monothiol glutaredoxins of Saccharomyces cerevisiae, regulate Aft1 nuclear localisation. We provide evidence of a negative regulation of Aft1 activity by Grx3 and Grx4. The Grx domain of both proteins played an important role in Aft1 translocation to the cytoplasm. This function was not, however, dependent on the availability of iron. Here we demonstrate that Grx3, Grx4 and Aft1 interact each other both in vivo and in vitro, which suggests the existence of a functional protein complex. Interestingly, each interaction occurred independently on the third member of the complex. The absence of both Grx3 and Grx4 induced a clear enrichment of G1 cells in asynchronous cultures, a slow growth phenotype, the accumulation of intracellular iron and a constitutive activation of the genes regulated by Aft1. The grx3grx4 double mutant was highly sensitive to the oxidising agents hydrogen peroxide and t-butylhydroperoxide but not to diamide. The phenotypes of the double mutant grx3grx4 characterised in this study were mainly mediated by the Aft1 function, suggesting that grx3grx4 could be a suitable cellular model for studying endogenous oxidative stress induced by deregulation of the iron homeostasis. However, our results also suggest that Grx3 and Grx4 might play additional roles in the oxidative stress response through proteins other than Aft1.

Grx5 is a yeast mitochondrial protein involved in ironsulfur biogenesis that belongs to a recently described family of monothiolic glutaredoxin-like proteins. No member of this family has been biochemically characterized previously. Grx5 contains a conserved cysteine residue (Cys-60) and a non-conserved one (Cys-117). In this work, we have purified wild type and mutant C60S and C117S proteins and characterized their biochemical properties. A redox potential of ؊175 mV was calculated for wild type Grx5. The pK a values obtained by titration of mutant proteins with iodoacetamide at different pHs were 5.0 for Cys-60 and 8.2 for Cys-117. When Grx5 was incubated with glutathione disulfide, a transient mixed disulfide was formed between glutathione and the cystein 60 of the protein because of its low pK a . Binding of glutathione to Cys-60 promoted a decrease in the Cys-117 pK a value that triggered the formation of a disulfide bond between both cysteine residues of the protein, indicating that Cys-117 plays an essential role in the catalytic mechanism of Grx5. The disulfide bond in Grx5 could be reduced by GSH but at a rate at least 20 times slower than that observed for the reduction of glutaredoxin 1 from E. coli, a dithiolic glutaredoxin. This slow reduction rate could suggest that GSH may not be the physiologic reducing agent of Grx5. The fact that wild type Grx5 efficiently reduced a glutathiolated protein used as a substrate indicated that Grx5 may act as a thiol reductase inside the mitochondria.