Yeast functional studies with

Mediators and coactivators. Transcriptional activators that act in a crude cell-free system often do not function with purified DNA, RNA polymerase, and the basal transcription factors as indicated in Eq. 28-5. Studies with yeast, Drosophila, and human cells revealed that additional large multisubunit complexes known as mediators are needed 272/346-348 A yeast mediator complex consists of 20 subunits.349-350b Many activator proteins bind to the DNA sequences known as enchancers, discussed in the next section. Mediator complexes may also interact with enhancer-bound activators. Individual proteins, such as the TAF subunits, that bind to and cooperate with activator proteins are often called coactivators.351... 

Efficient error-free nucleotide insertion opposite an AAF-dG adduct can be catalyzed by Polr in vitro. The human Polr is more efficient in subsequent extension synthesis as compared to the yeast Polq. If the error-free translesion synthesis activity of Polr is utilized in cells in response to AAF-dG adducts, this polymerase would function to suppress AAF-induced mutagenesis. In one study with yeast cells, both an error-free bypass role and a frameshift mutagenesis role of Polq were reported. Hence, it is still unknown about the contribution of Polq to AAF-induced mutagenesis. Opposite a template AAF-dG adduct, human Poll is able to insert predominantly a C in vitro. Subsequent extension synthesis, however, was not observed. 

Defects in mitochondrial iron transport and utilization can result in mitochondrial iron overload. There is extensive iron accumulation in erythroblast mitochondria of both patients with X-linked sideroblastic anemia due to defective erythroid-speeifie 5-aminolevulinic acid synthase (eALAS) and those with ring sideroblasts associated with myelodysplastic syndrome. Mitochondrial iron overload has also been documented in patients with Friedreich s ataxia with defective frataxin" and in those with sideroblastic anemia with ataxia from defects in the Fe-S transporter ABC7. In addition, studies with yeast, the best studied eukaryotic model of Fe-S cluster synthesis, showed that defects in any of the enzymes of the Fe-S cluster assembly pathway caused mitochondrial iron accumulation and lack of normal mitochondrial function. ... 

Studies with yeast, heart, and brain have shown that concentrations of intermediates within the glycolytic pathway often follow an oscillating function. Continuous spectrophotometric recording techniques for determining the NAD" /NADH ratio in cell-free extracts first revealed oscillations of the NADH level in these systems. These studies then led to the discovery of glycolytic oscillations in yeast cell and cell-free extracts, beef heart extracts, rat skeletal muscle extracts, and in ascites tumor cells, with concentrations of intermediates varying in the range between 10 and 10 M (Chance et al., 1973). 

The bakers yeast Saccharomyces cerevisiae represents probably the currently best researched and understood eukaryotic organism. Most of its genes have been knocked out for functional studies, and hence a vast variety of mutant strains as well as tools for their manipulation are available. This in concordance with short generation times and a growth medium that is comparable in cost and complexity to bacterial media has made the yeast system a frequent choice for the evaluation of transmembrane carriers. 

Vitamins are required for satisfactory development or function of most yeasts. Wickerham (177) devised a complete yeast medium which included eight vitamins biotin, pantothenic acid, inositol, niacin, p-aminobenzoic acid, pyridoxine, thiamine, and riboflavin. The concentrations of these growth factors varied widely with inositol in the greatest concentration and biotin in trace amounts. Many of these vitamins are considered major growth factors for yeast multiplication and development, as noted in several studies and reviews (178, 179, 180, 181, 182). Generally, the benefit of adding vitamins to musts and wines has not been established as a normal winery practice. This lack of response is because vitamins occur naturally in sufficient quantities in grapes and are produced by yeasts themselves (3). 

Like other nuclear receptors (e.g., steroid hormone receptors, thyroid hormone receptors) the PPARs function as ligand-activated transcription factors. As illustrated in Fig. 1 (see color insert) individual PPARs function as dimers with members of the retinoid X receptor (RXR) family (23). Evidence for an interaction of PPARs with RXRs includes co-expression studies that were performed with yeast lacking endogenous nuclear receptors (24). 

Isolation of the immunophilin cDNAs has rendered possible the production and the structure-function study of these proteins by site-directed mutagenesis. In addition, yeast strains have been constructed where the endogenous ligands for CsA and FK506 have been replaced with their human counterparts. Such strains provide convenient assay systems for the stracture-activity studies of putative immunosuppressants (Koltin et al, 1991). The advances in immunosuppression are yet another salient example of the contributions of molecular biology to drug design and development. 

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