The molecular events involved in the induction of petite yeast mutants by fluorinated pyrimidines

Flucytosine antagonism of azole activity versus Candida glabrata: role of transcription factor Pdr1 and multidrug transporter Cdr1

Infections with the opportunistic yeast Candida glabrata have increased dramatically in recent years. Antifungal therapy of yeast infections commonly employs azoles, such as fluconazole (FLC), but C. glabrata frequently develops resistance to these inhibitors of ergosterol biosynthesis. The pyrimidine analog flucytosine (5-fluorocytosine [5FC]) is highly active versus C. glabrata but is now rarely used clinically due to similar concerns over resistance and, a related concern, the toxicity associated with high doses used to counter resistance. Azole-5FC combination therapy would potentially address these concerns; however, previous studies suggest that 5FC may antagonize azole activity versus C. glabrata. Here, we report that 5FC at subinhibitory concentrations antagonized the activity of FLC 4- to 16-fold versus 8 of 8 C. glabrata isolates tested. 5FC antagonized the activity of other azoles similarly but had only indifferent effects in combination with unrelated antifungals. Since azole resistance in C. glabrata results from transcription factor Pdr1-dependent upregulation of the multidrug transporter gene CDR1, we reasoned that 5FC antagonism might be similarly mediated. Indeed, 5FC-FLC antagonism was abrogated in pdr1Δ and cdr1Δ strains. In further support of this hypothesis, 5FC exposure induced CDR1 expression 6-fold, and this upregulation was Pdr1 dependent. In contrast to azoles, 5FC is not a Cdr1 substrate and so its activation of Pdr1 was unexpected. We observed, however, that 5FC exposure readily induced petite mutants, which exhibit Pdr1-dependent CDR1 upregulation. Thus, mitochondrial dysfunction resulting in Pdr1 activation is the likely basis for 5FC antagonism of azole activity versus C. glabrata.

Cytosine deaminase MX cassettes as positive/negative selectable markers in Saccharomyces cerevisiae

We describe positive/negative selectable cytosine deaminase MX cassettes for use in Saccharomyces cerevisiae. The basis of positive selection for cytosine deaminase (Fcy1) activity is that (a) fcy1 strains are unable to grow on medium containing cytosine as a sole nitrogen source and (b) fcy1 ura3 strains are unable to grow on medium containing cytosine as the sole pyrimidine source. Conversely, as 5-fluorocytosine (5FC) is toxic to cytosine deaminase-producing cells, fcy1 strains are resistant to 5FC. FCY1MX and FCA1MX cassettes, containing open reading frames (ORFs) of S. cerevisiae FCY1 and Candida albicans FCA1, respectively, were constructed and used to disrupt targeted genes in S. cerevisiae fcy1 strains. In addition, new direct repeat cassettes, kanPR, FCA1PR, FCY1PR and CaURA3PR, were developed to allow efficient deletion of target genes in cells containing MX3 repeats. Finally, the FCY1- and FCA1MX3 or PR direct repeat cassettes can be readily recycled after 5FC counter-selection on both synthetic and rich media.

Development of Saccharomyces cerevisiae as a model pathogen. A system for the genetic identification of gene products required for survival in the mammalian host environment

Saccharomyces cerevisiae, a close relative of the pathogenic Candida species, is an emerging opportunistic pathogen. An isogenic series of S. cerevisiae strains, derived from a human clinical isolate, were used to examine the role of evolutionarily conserved pathways in fungal survival in a mouse host. As is the case for the corresponding Candida albicans and Cryptococcus neoformans mutants, S. cerevisiae purine and pyrimidine auxotrophs were severely deficient in survival, consistent with there being evolutionary conservation of survival traits. Resistance to the antifungal drug 5-fluorocytosine was not deleterious and appeared to be slightly advantageous in vivo. Of mutants in three amino acid biosynthetic pathways, only leu2 mutants were severely deficient in vivo. Unlike the glyoxylate cycle, respiration was very important for survival; however, the mitochondrial genome made a respiration-independent contribution to survival. Mutants deficient in pseudohyphal formation were tested in vivo; flo11Delta mutants were phenotypically neutral while flo8Delta, tec1Delta, and flo8Delta tec1Delta mutants were slightly deficient. Because of its ease of genetic manipulation and the immense S. cerevisiae database, which includes the best annotated eukaryotic genome sequence, S. cerevisiae is a superb model system for the identification of gene products important for fungal survival in the mammalian host environment.

Lack of carcinogenicity and increased survival in F344 rats treated with 5-fluorouracil for two years

The carcinogenicity of 5-fluorouracil (5-FU), a compound employed as an antineoplastic drug, was investigated in F344 rats of both sexes. 5-FU was administered to groups of 50 male and 50 female rats ad lib. for 104 weeks, added to drinking water at concentrations of 0 (control), 62 and 125 ppm, these dose levels being selected on the basis of results of a 13-week subchronic toxicity study. Body weight gains were slightly depressed in the 125 ppm group of both sexes. While not statistically significant in females, final survival rates at week 111 in the 125 ppm group of both sexes were higher than those in the control group, suggesting an ability of 5-FU to prolong the lifespan. Histopathologically, a decreased incidence of islet cell adenomas in males and increased incidences of pituitary gland adenomas and pheochromocytomas in females were observed in the 62 ppm group without dose dependence. There was no significant induction of any other neoplastic or non-neoplastic lesions. These results indicate a lack of carcinogenicity of 5-FU under the present experimental conditions using rats.

Primary mitochondrial activity of gossypol in yeast and mammalian cells

Gossypol showed primary antimitochondrial activity in yeast cells in that the drug (1) inhibited growth of cells utilizing mitochondrial substrates as carbon and energy sources, and (2) selectively inhibited mitochondrial protein synthesis. Primary antimitochondrial activity was demonstrated in guinea-pig keratinocytes (GPK) by early arrest of growth and loss of viability in medium with glutamine (a mitochondrial substrate) as carbon and energy source compared with cells utilizing glucose. Gossypol depressed oxygen uptake directly in respiring cells. Gossypol interacted with the known antimitochondrial agents ethidium bromide and 5-fluorouracil (FU), potentiating the activity of FU but reversing that of ethidium bromide in yeast and GPK. Also, the activity of the mitochondrial inhibitor oligomycin was reversed by the presence of gossypol in yeast cells but not tested in GPK. The uptake and retention of the mitochondria-specific dye rhodamine 123 were much depressed by gossypol in GPK. Gossypol showed little or no inhibitory effects in yeast or GPK in the presence of ethanol (0.2-0.5%). The drug was not mutagenic with respect to the yeast mitochondrial system. It was tentatively suggested that mitochondrial perturbation could explain the antifertility effect of gossypol if it is assumed that mitochondria have a special role to play in spermatogenesis and sperm motility, making these tissues more sensitive to mitochondrial inhibitors than somatic cells.

The role of the nuclear gene "mitochondrial mutability control" (MMC1) in the process of mutability of the mitochondrial genome by different mutagens in Saccharomyces cerevisiae

The accumulation of respiratory deficient (RD) mutants in Saccharomyces cerevisiae depended upon the mutagens used and upon the presence of the nuclear gene previously identified as MMC1 (one) which we showed to control the spontaneous and the erythromycin-induced RD mutability. In this paper data are reported about the accumulation of RD mutants in the presence of manganous ions (Mn++) and UV which was higher in the mmc1 (one) than in MMC1 strains. We found that the characters 'low spontaneous' and 'low induced' RD mutability by erythromycin, manganous ions and UV, were controlled by the same genetic determinant. In the presence of manganous ions, also the frequency of antibiotic resistant mutants capR and eryR was higher in the mmc1 strains. Moreover, the accumulation of RD mutants in the presence of berenil, 5-fluorouracil and basic fuchsin was higher in the mmc1 than in MMC1 strains. In contrast, RD mutants accumulation by acriflavine and ethidium bromide treatments did not appear affected by the MMC1 genetic constitution.

Comparison of petite induction in yeast by acridines, ethidium and their photoaffinity probes

The production of petite mutations by different acridine analogs was studied in Saccharomyces cerevisiae. Compounds with amino substituents at the 2 and 3 positions of the acridine nucleus and methylation at position 10 were effective for petite induction in growing cells but not in resting cells, while those with chloro, nitro and methoxy substituents were not effective in either resting or growing cells. Photosensitive azido derivatives of the acridines were tested to evaluate the role of covalent drug attachment for mutagenesis in resting cells. Photolysis of resting cells with 9-axido, 3-azido-6-amino-, 9-azido-10-methyl-, or 3-azido-6-amino-10-methyl-acridine was highly toxic. 3-Azido-6-amino-acridine, and especially 3-azido-10-methyl-, and 3-azido-6-amino-10-methyl-acridine, were effective petite inducers in resting cells. Thus, the photosensitive (azido) group at position 9 produced only cell killing while the azido group at position 3 and/or 6 led to effective petite induction in resting cells. In contrast, petite induction was observed only for growing cells, for dark control experiments with these compounds or with the monoazide precursor compounds.

Mutants of yeast specifically resistant to petite induction by fluorinated pyrimidines

Induction of the cytoplasmic petite mutation in yeast by 5-fluorouracil (5FU) and 5-fluorocytosine (5FC) is known to depend on the incorporation of 5FU into some species of RNA; 5FC is active only following deamination to 5FU. Several mutants ahve now been isolated which are resistant to petite mutagenesis by 5FU but remain sensitive to growth inhibition by this analogue. They fall into two classes: those in class I are also resistant to mutagenesis by 5FC, while class II mutants retain partial sensitivity to the latter agent. The growth of both classes is sensitive to 5FC. The behavior of class II mutants requires that exogenous 5FU is specifically excluded from the site of synthesis of the target RNA involved in petite mutagenesis, while 5FC has access to it. The most likely explanation is that the RNA concerned is synthesized in the mitochondria, and that the mitochondrial membranes of class II mutants are impermeable to 5FU but not 5FC. This is supported by the finding that the membrane-active agent dimethylsulfoxide restored 5FU sensitivity to this class of mutants. No such effect was observed with class I mutants, and these are thought to have altered mitochondrial RNA-synthesizing systems which are unable to recognize fluorinated nucleotides.

The conditions required for the induction of petite yeast mutants by fluorinated pyrimidines

Cytoplasmic petite mutagenesis by 5-fluorouracil (5FU) was prevented by temperature sensitive mutations which blcoked either nuclear transcription or cytoplasmic translation. However, 5FU was also ineffective in resting cells and in cells exposed to alpha-mating factor, showing that cell division or nuclear DNA synthesis is required for the mutagenic event to take place. In addition, the mutagenic effect of 5FU was completely prevented by daunomycin, and since this agent preferentially inhibits respiratory growth and was shown to selectively block RNA synthesis in the mitochondria, it was concluded that petite mutagensis resulted from incorporation of 5FU into mitochondrial RNA. Since inhibition of mitochondrial protein synthesis by erythromycin had little immediate effect on the mutagenicity of 5FU, it was deduced that the RNA in question is not directly involved in mitochondrial translation, and may have a regulatory function.