Localization of mucidin-resistant locus muc3 on mitochondrial DNA with respect to ubiquinol-cytochrome c reductase deficient box loci. Locus muc3 is allelic to box2

Synthetic lethal interaction between the pel1 and op1 mutations in Saccharomyces cerevisiae

The Saccharomyces cerevisiae mutant strain containing the op1 mutation affecting the function of a mitochondrial ATP/ADP translocator has been crossed to the pel1 and crd1 mutants deficient in the biosynthesis of mitochondrial phosphatidylglycerol (PG) and cardiolipin (CL). Using tetrad analysis of diploids issued from corresponding crosses a synthetic lethal interaction has been observed between the op1 and pel1 mutations resulting in the lack of growth of a corresponding double mutant on minimal medium containing glucose. The op1 pel1 double mutant also displayed a decreased susceptibility to fluconazole and a compromised growth even in complex medium containing glucose. The viability of mutant cells was strongly reduced, corresponding to <30 % and 10 % of colony-forming units observed after growth in complex and minimal medium, respectively. A lower viability of the double mutant in minimal medium was accompanied by an increased formation of mitochondrial petite mutants (as determined by mtDNA rescue into diploid cells). The results indicate that in the simultaneous absence of mitochondrial anionic phospholipids (PG plus CL) and ATP/ADP exchange across the inner mitochondrial membrane the yeast mitochondrial functions are severely limited, leading to a strongly compromised cell multiplication. Since under similar conditions the op1 crd1 double mutant was able to grow on minimal medium this deleterious effect of anionic phospholipid deficiency could be at least partially substituted by PG accumulated in the cardiolipin deficient delta crd1 mutant cells.

Cross-resistance to strobilurin fungicides in mitochondrial and nuclear mutants ofSaccharomyces cerevisiae

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.

Random mutant generation and its utility in uncovering structural and functional features of cytochrome b in Saccharomyces cerevisiae

The generation of random mutations in the mitochondrial cytochrome b gene of Saccharomyces cerevisiae has been used as a most fruitful means of identifying subregions that play a key role in the bc1 complex mechanism, best explained by the protonmotive Q cycle originally proposed by Peter Mitchell. Selection for center i and center o inhibitor resistance mutants, in particular, has yielded much information. The combined approaches of genetics and structural predictions have led to a two-dimensional folding model for cytochrome b that is most compatible with current knowledge of the protonmotive Q cycle. A three-dimensional model is emerging from studies of distant reversions of deficient mutants. Finally, interactions between cytochrome b and the other subunits of the bc1 complex, such as the iron-sulfur protein, can be affected by a single amino acid change.

Isolation and RNA sequence analysis of cytochrome b mutants resistant to funiculosin, a center i inhibitor of the mitochondrial ubiquinol-cytochrome c reductase in Saccharomyces cerevisiae

Funiculosin is a well-known inhibitor of the mitochondrial respiratory chain, probably acting at the ubiquinone reducing site or center i of QH2-cytochrome c reductase. We report here the isolation, mapping and RNA sequence analysis of yeast apo-cytochrome b mutants resistant to this inhibitor. Funiculosin-resistance was found to be conferred, in 4 independent isolates, upon replacement of a leucine residue by phenylalanine in position 198 of the cytochrome b polypeptide chain.

Mitochondrial cytochrome b genes with a six-nucleotide deletion or single-nucleotide substitutions confer resistance to antimycin A in the yeast Kluyveromyces lactis

Extrachromosomal mutants resistant to antimycin, from the yeast Kluyveromyces lactis, have been isolated, genetically characterized, and assigned to two specific genetic loci (Brunner et al., 1987). In the present work the cytochrome b nucleotide sequence from six of these mutants was determined. Five mutants had single point mutations, corresponding to transversions. In one mutant, a six-base-pair deletion, beginning at nucleotide 689, was observed. The amino acid sequence derived from the coding strand showed that, in three independent antimycin-resistant mutants, a change of asparagine 31 into lysine took place (two of these mutants are also resistant to diuron). Two other mutants showed a change from lysine 228 into isoleucine (or methionine). Leucine 230, isoleucine 231, and threonine 232, were lost in the deletion mutant and were replaced by serine.

Extrachromosomal genetics in the yeast Kluyveromyces lactis. Isolation and characterization of antimycin-resistant mutants

Antimycin-resistant (AR) mutants of the yeast Kluyveromyces lactis, obtained either spontaneously or after manganese treatment, were isolated and genetically characterized. Most of the mutants obtained after manganese mutagenesis and two spontaneous mutants, tolerated high antimycin concentrations (more than 10 micrograms/ml) and were extrachromosomal. One mutant which grew only in low antimycin (1 microgram/ml) showed a Mendelian type of inheritance. The extrachromosomal mutants could be assigned to at least two genetic loci (ARI and ARII). Mutants representative of these two groups showed increased resistance to the antibiotic when the respiration of whole cells or mitochondria was studied. Extrachromosomal mutants of Saccharomyces cerevisiae resistant to antimycin were also induced with manganese, isolated and characterized. Comparative studies of the antimycin-resistant mutants of K. lactis and S. cerevisiae permitted the following observations: a) K. lactis is more resistant to antimycin, funiculosin, mucidin and diuron than S. cerevisiae, as are the AR mutants; b) K. lactis shows correlated sensitivity to funiculosin differing in this aspect from S. cerevisiae; c) the antimycin-resistant mutants of K. lactis belonging to group II (ARII) were also resistant to diuron, tolerating concentrations of more than 200 micrograms/ml; d) all extrachromosomal antimycin-resistant-mutants of S. cerevisiae and some of the AR mutants of K. lactis were more sensitive to mucidin than the wild type.

DNA sequence analysis of diuron-resistant mutations in the mitochondrial cytochrome b gene of Saccharomyces cerevisiae

Diuron (3-[3,4-dichlorophenyl]-1,1-dimethylurea), an inhibitor of mitochondrial respiration, blocks the yeast respiratory chain between cytochrome b and c1. Diuron-resistant mutants of Saccharomyces cerevisiae have been selected and several mutations localized to the mitochondrial cytochrome b gene. The present paper identifies specific DNA base changes within the cytochrome b gene conferring diuron-resistance. DNA sequence analysis was done utilizing primer extension of crude mitochondrial RNA preparations in the presence of reverse transcriptase. Five independent diuron-resistant mutations have been sequenced.

Genetic mapping of nuclear mucidin resistance mutations in Saccharomyces cerevisiae. A new pdr locus on chromosome II

In the yeast Saccharomyces cerevisiae, two nuclear pleiotropic drug resistance mutations pdr3-1 (former designation mucPR) and pdr3-2 (former designation DRI9/T7) have been selected as resistant to mucidin and as resistant to chloramphenicol plus cycloheximide, respectively. The pdr3 mutations were found not to affect the plasma membrane ATPase activity measured in a crude membrane fraction. Meiotic mapping using strains with standard genetic markers revealed that mutation pdr3-1 is centromere linked on the left arm of chromosome II at a distance of 5.9 +/- 3.3 cM from its centromere and 11.6 +/- 3.1 cM from the marker pet9. The centromere linked pdr3-2 mutation exhibited also genetic linkage to pet9 with a map distance of 9.8 +/- 3.2 cM. These results indicate that pdr3-1 and pdr3-2 are alleles of the same pleiotropic drug resistance locus PDR3 which is involved in the control of the plasma membrane permeability in yeast.

The cytochrome oxidase subunit I split gene in Saccharomyces cerevisiae: genetic and physical studies of the mtDNA segment encompassing the 'cytochrome b-homologous' intron

We have constructed a refined genetic and physical map of 38 oxi3 mutations. With the help of the rho- clones derived from 'short' and 'long' genes, pairwise crosses between mutants, estimations of their reversion frequencies and analyses of mitochondrially synthesized proteins, we have characterized and localized several mutants in the exon A4 and in the intron aI4. We present genetic and physical evidence that in the 'long' gene the exon A5 is split into at least three quite distinct exons, A5-1, A5-2 and A5-3 where numerous mutations are localized. We suggest that a novel 56 Kd polypeptide, which accumulates in some cis-dominant oxi3- mutants results from the translation of the upstream exons and the downstream aI4 intron.

Spectral properties of cytochrome b-561 and cytochrome b-565 in mucidin-resistant mutants of Saccharomyces cerevisiae

The oxidation of NADH in submitochondrial particles isolated from MUC1, MUC2 and MUC3 mucidin-resistant mutants of Saccharomyces cerevisiae is specifically resistant to mucidin. Extra reduction of cytochrome b-565 induced by mucidin is demonstrated in all tested mucidin-resistant mutants. Red shift of cytochrome b-561 is induced by mucidin in two independent MUC3 mutants. In MUC1 and MUC2 mutants, the red shift is not induced by mucidin, while that promoted by antimycin A and 2-n-heptyl-4-hydroxyquinoline N-oxide are normal. It is concluded that the extra reduction of cytochrome b-565 and the red shift of cytochrome b-561 elicited by mucidin can be largely dissociated from the overall inhibition of the electron flow by distinct mucidin-resistant mutations in different exons of the split mitochondrial gene of cytochrome b.

Modification of the spectral properties of cytochrome b in mutants of Saccharomyces cerevisiae resistant to 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Mapping at two distinct genetic loci of the split mitochondrial gene of cytochrome b

The effects of five inhibitors of the cytochrome bc1 complex: 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), 2-n-heptyl-4-hydroxyquinoline-N-oxide (HpHOQnO), antimycin A, funiculosin and mucidin were measured in submitochondrial particles of strains of the yeast Saccharomyces cerevisiae belonging to two classes of diuron-resistant mutants Diu 1 and Diu 2 which are modified in different exons of the split mitochondrial gene of cytochrome b. 1. The oxidation of NADH and of cytochrome b-561 exhibits a similar resistance to diuron and HpHOQnO in Diu 1 and Diu 2 mutants. 2. No extra reduction of cytochrome b-561 and cytochrome b-565 is observed in the presence of diuron and HpHOQnO. 3. Both Diu 1 and Diu 2 mutants exhibit the red shift of cytochrome b-561 induced by concentrations of HpHOQno 2 -- 3-times higher than those required in the parental strains. 4. The spectral and respiratory effects of antimycin A, funiculosin and mucidin and generally similar in the diuron-resistant mutants and in their parental strains. However a cross-resistance between diuron and antimycin A is indicated in one Diu 2 mutant. 5. From the combined genetic and biochemical data it is concluded that the interaction of diuron and HpHOQnO with cytochrome b is mediated by at least two specific amino acids located apart in the central region of the apocytochrome b peptide coded by mitochondrial DNA. These two amino acids control tightly the extra reduction of cytochromes b-565 and b-561 as well as the flow of electrons through the bc1 complex. However the binding of HpHOQnO required for the expression of the red shift of cytochrome b-561 is only slightly affected by the diu-1 and diu-2 mutations.