Cloning by Gene Amplification of Two Loci Conferring Multiple Drug Resistance in Saccharomyces

Yeast DNA fragments that confer multiple drug resistance when amplified were isolated. Cells containing a yeast genomic library cloned in the high copy autonomously replicating vector, YEp24, were plated on medium containing cycloheximide. Five out of 100 cycloheximide-resistant colonies were cross-resistant to the unrelated inhibitor, sulfometuron methyl, due to a plasmid-borne resistance determinant. The plasmids isolated from these resistant clones contained two nonoverlapping regions in the yeast genome now designated PDR4 and PDR5 (for pleiotropic drug resistant). PDR4 was mapped to chromosome XIII, 31.5 cM from LYS7 and 9 cM from the centromere. PDR4 was mapped to chromosome XV between ADE2 and H1S3. Genetic analysis demonstrated that at least three tightly linked genes (PDR5, PDR2 and SMR3) that mediate resistance to inhibitors are located in this region. Insertion mutations in the either PDR4 or PDR5 genes are not lethal, but the insertion in PDR5 results in a drug-hypersensitive phenotype.     

Cross-resistance to strobilurin fungicides in mitochondrial and nuclear mutants of<Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

In yeast the resistance to kresoxim-methyl and azoxystrobin, like the resistance to strobilurin A (mucidin) is under the control of both mitochondrial cob gene and the PDR network of nuclear genes involved in multidrug resistance. The mucidin-resistant mucl (G137R) and muc2 (L275S) mutants of Saccharomyces cerevisiae containing point mutations in mtDNA were found to be cross-resistant to kresoxim-methyl and azoxystrobin. Cross-resistance to all three strobilurin fungicides was also observed in yeast transformants containing gain-of-function mutations in the nuclear PDR3 gene. On the other hand, nuclear mutants containing disrupted chromosomal copies of the PDR1 and PDR3 genes or the PDR5 gene alone were hypersensitive to kresoxim-methyl, azoxystrobin and strobilurin A. The frequencies of spontaneous mutants selected for resistance either to kresoxim-methyl, azoxystrobin or strobilurin A were similar and resulted from mutations both in mitochondrial and nuclear genes. The results indicate that resistance to strobilurin fungicides, differing in chemical structure and specific activity, can be caused by the same molecular mechanism involving changes in the structure of apocytochrome b and/or increased efflux of strobilurins from fungal cells.     

Sensitivity to lovastatin of Saccharomyces cerevisiae strains deleted for pleiotropic drug resistance (PDR) genes

The use of statins is well established in human therapy, and model organisms such as Saccharomyces cerevisiae are commonly used in studies of drug action at molecular and cellular levels. The investigation of the resistance mechanisms towards statins may suggest new approaches to improve therapy based on the use of statins. We investigated the susceptibility to lovastatin of S. cerevisiae strains deleted for PDR genes, responsible for exporting hydrophobic and amphiphilic drugs, such as lovastatin. Strains deleted for the genes tested, PDR1, PDR3, PDR5 and SNQ2, exhibited remarkably different phenotypes, with deletion of PDR5 causing the highest sensitivity to lovastatin. The study helped clarifying which pdr mutants to use in studies of physiological actions of statins in yeast.     

Responses of pathogenic and nonpathogenic yeast species to steroids reveal the functioning and evolution of multidrug resistance transcriptional networks

Steroids are known to induce pleiotropic drug resistance states in hemiascomycetes, with tremendous potential consequences for human fungal infections. Our analysis of gene expression in Saccharomyces cerevisiae and Candida albicans cells subjected to three different concentrations of progesterone revealed that their pleiotropic drug resistance (PDR) networks were strikingly sensitive to steroids. In S. cerevisiae, 20 of the Pdr1p/Pdr3p target genes, including PDR3 itself, were rapidly induced by progesterone, which mimics the effects of PDR1 gain-of-function alleles. This unique property allowed us to decipher the respective roles of Pdr1p and Pdr3p in PDR induction and to define functional modules among their target genes. Although the expression profiles of the major PDR transporters encoding genes ScPDR5 and CaCDR1 were similar, the S. cerevisiae global PDR response to progesterone was only partly conserved in C. albicans. In particular, the role of Tac1p, the main C. albicans PDR regulator, in the progesterone response was apparently restricted to five genes. These results suggest that the C. albicans and S. cerevisiae PDR networks, although sharing a conserved core regarding the regulation of membrane properties, have different structures and properties. Additionally, our data indicate that other as yet undiscovered regulators may second Tac1p in the C. albicans drug response.     

Molecular mechanisms of deoxynivalenol resistance in the yeast<Emphasis Type="Italic">Saccharomyces cerevisiae</Emphasis>

The production of trichothecene toxins is a suspected virulence mechanism of several plant pathogenic fungi. This hypothesis has been confirmed forGibberella zeae (Fusarium graminearum) by gene disruption experiments, suggesting in turn, that resistance against the fungal toxin is a relevant component ofFusarium resistance of the host plant. Our goal is therefore to identify molecular mechanisms of trichothecene resistance. Using yeast as a model system we have found the following resistance mechanisms and genes: a) reduced uptake of deoxynivalenol (PDR5), b) toxin modification and reduction of toxicity (AYT1), and c) formation of a resistant toxin target (RPL3). Homologous plant genes exist and are attractive candidates forFusarium resistance genes.     

RNA processing in vitro produces mature 3'' ends of a variety of Saccharomyces cerevisiae mRNAs.

Ammonium sulfate fractionation of a Saccharomyces cerevisiae whole-cell extract yielded a preparation which carried out correct and efficient endonucleolytic cleavage and polyadenylation of yeast precursor mRNA substrates corresponding to a variety of yeast genes. These included CYC1 (iso-1-cytochrome c), HIS4 (histidine biosynthesis), GAL7 (galactose-1-phosphate uridyltransferase), H2B2 (histone H2B2), PRT2 (a protein of unknown function), and CBP1 (cytochrome b mRNA processing). The reaction processed these pre-mRNAs with varying efficiencies, with cleavage and polyadenylation exceeding 70% in some cases. In each case, the poly(A) tail corresponded to the addition of approximately 60 adenosine residues, which agrees with the usual length of poly(A) tails formed in vivo. Addition of cordycepin triphosphate or substitution of CTP for ATP in these reactions inhibited polyadenylation but not endonucleolytic cleavage and resulted in accumulation of the cleaved RNA product. Although this system readily generated yeast mRNA 3' ends, no processing occurred on a human alpha-globin pre-mRNA containing the highly conserved AAUAAA polyadenylation signal of higher eucaryotes. This sequence and adjacent signals used in mammalian systems are thus not sufficient to direct mRNA 3' end formation in yeast. Despite the lack of a highly conserved nucleotide sequence signal, the same purified fraction processed the 3' ends of a variety of unrelated yeast pre-mRNAs, suggesting that endonuclease cleavage and polyadenylation may produce the mature 3' ends of all mRNAs in S. cerevisiae.