Genetic analysis of mitochondrial rho-mutability in Saccharomyces. IV. Relation between spontaneous rho-mutability and mitotic stability in disomic Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae the disomy for chromosome XIV resembles the previously described disomy for chromosome IV in that it leads to a significant decrease in spontaneous rho- mutability. The nuclear srm1 mutation, reducing spontaneous rho- mutability, diminishes significantly the mitotic disome stability. So, the mechanisms of spontaneous rho- mutagenesis and mitotic disome stability seem to compete for the function affected by the srm1 mutation.     

Genetic analysis of the mitochondrial rho-mutability in Saccharomyces. II. Disomy for chromosome IV and spontaneous rho-mutability

The disomy for chromosome IV in the strains studied led to: reduction in the red pigmentation of ade1 mutant colonies; a decrease in spontaneous rho- mutant frequency, and impairment of sporulation in hybrids descended from disomic parents. The nuclear srm1 mutation decreasing the spontaneous rho- mutability promoted the spontaneous extra chromosome loss in the disomics for chromosome IV. This result suggests a close connection between the spontaneous rho- mutability and mitotic chromosome stability.     

Genetic analysis of mitochondrial rho-mutability in Saccharomyces yeasts. VI. Quantitative characteristics of the effect of mutation srm5 on the mitochondrial stability of natural and recombinant genetic structures

The srm5 mutation diminishes the spontaneous rho- mutation rate by an order of magnitude. Frequency of rho- mutations is 500 times lower in homozygous cultures, as compared with those of normal SRM+/SRM+ diploids. The rate of spontaneous loss of extra chromosome IV is about 25 times higher in srm5 disomes, as compared with SRM+ ones. Haploid srm1 srm5 transformants loose recombinant circular minichromosomes spontaneously about 4 times more frequently than srm1SRM5 cells. The data presented suggest that general control of mitotic stability of different (mitochondrial and nuclear, nuclear as well as recombinant) genetic structures operates in Sacch. cerevisiae. Autonomously replicating sequences (ARS elements) seem to be involved in this mechanism.     

Genetic analysis of mitochondrial rho-mutability in Saccharomyces. III. Comparative analysis of the effect of various nuclear srm mutations and disomy for chromosome IV on rho-mutagenesis

Different combinations of modifying genes which enhance the rho- mutability of haploid yeast cells are shown to be suppressible by the srm1, srm2, srm3 mutations and by the disomy for chromosome IV. The srm1 mutation leads to dramatic decrease in both the spontaneous and ethidium-bromide induced rho- mutability. Other srm mutations studied and the disomy appear to cause relatively moderate quantitative changes in the spontaneous rho- mutation rate and to have no significant effect on mutation induction by ethidium bromide. Neither additivity nor synergism was revealed by the analysis of the interaction between the srm mutations. We suggest that in Saccharomyces an efficient mechanism of the rho- mutagenesis operates which can be directly affected by the srm1 mutation and more or less modified by other srm mutations under study and by the disomy for chromosome IV.     

Genetic analysis of the mutant RD-50 in Saccharomyces cerevisiae. The role of nuclear and mitochondrial mutation

Inheritance of the mutant phenotype of respiratory deficience in RD-50 strain was studied. The deficience can be restored, giving respiratory sufficience, in crosses with rho0 testers. The "restorable" phenotype of the mutant, named RDc+, was shown to be determined by a nuclear pet-like mutation (pet50). The restorable RDc+ phenotype is stabilized in the presence of the pet50 allele, but can remain as such in the presence of the wild-type allele PET50. Restoration also takes place in cytoductants with the nucleus of kar partner. In order to explain the behaviour of the mitochondrial mutation mit50, we suppose it to be a microdeletion, capable of reversion, due to integration of a putative mt episome. Some features of the nuclear mutation pet50, particularly, its segregation in mitotic progeny of some revertants to respiratory competence point to its peculiarity. We suppose the mutation pet50 to be an insertion into the chromosomal PET50 gene. This insertion may be excised, remaining within the cell in the free state for some time, and then either eliminate or reintegrate into the chromosome.     

Genetic analysis of spontaneous suppressors of the pho85 mutation in the yeast Saccharomyces cerevisiae

Chaperones are known to play an important role in complexation of cyclin-dependent kinases with cyclins. In yeast cells growing in the presence of phosphate, cyclin-dependent kinase Pho85p and cyclin Pho80p form a complex and phosphorylate activator Pho4p. As a result, Pho4p is exported from the nucleus, and the PHO5 gene is not transcribed. The mutations suppressing the pho85 mutation were analyzed in order to identify genes which code for chaperones involved in the formation of the Pho80p-Pho85p complex in the presence of environmental phosphate. Dominant mutations DSP1, DSP2, and DSP4-6 were found. It is shown that the DSP1 gene is 2.1 cM away from the PHO85 gene on chromosome XVI and probably coincides with the EGD1 gene coding for a chaperone.     

Nuclear mutations in Saccharomyces cerevisiae which increase the spontaneous mutation frequency in mitochondrial DNA.

Summary Fourteen mutants have been identified in which the frequency of spontaneous mutations in mitochondrial DNA is increased. As well as increasing the frequency of mutations to resistance to erythromycin, oligomycin and spiramycin, all the mutants also show changes in the frequency of spontaneous petite induction. None of the mutants has any effect on the frequency of spontaneous nuclear mutations. Nine of the mutants are in one complementation group and five are in another. The phenotype of both groups is caused by a single nuclear mutation.     

Polygenic mutation in Drosophila melanogaster: Mapping spontaneous mutations affecting sensory bristle number

Our ability to predict long-term responses to artificial and natural selection, and understand the mechanisms by which naturally occurring variation for quantitative traits is maintained, depends on detailed knowledge of the properties of spontaneous polygenic mutations, including the quantitative trait loci (QTL) at which mutations occur, mutation rates, and mutational effects. These parameters can be estimated by mapping QTL that cause divergence between mutation-accumulation lines that have been established from an inbred base population and selected for high and low trait values. Here, we have utilized quantitative complementation to deficiencies to map QTL at which spontaneous mutations affecting Drosophila abdominal and sternopleural bristle number have occurred in 11 replicate lines during 206 generations of divergent selection. Estimates of the numbers of mutations were consistent with diploid per-character mutation rates for bristle traits of 0.03. The ratio of the per-character mutation rate to total mutation rate (0.023) implies that >2% of the genome could affect just one bristle trait and that there must be extensive pleiotropy for quantitative phenotypes. The estimated mutational effects were not, however, additive and exhibited dependency on genetic background consistent with diminishing epistasis. However, these inferences must be tempered by the potential for epistatic interactions between spontaneous mutations and QTL affecting bristle number on the deficiency-bearing chromosomes, which could lead to overestimates in numbers of QTL and inaccurate inference of gene action.     

A temperature sensitive mitochondrial mutation of Saccharomyces cerevisiae.

A mitochondrially inherited temperature sensitive respiratory deficient mutant of yeast has been isolated. Detection of nuclear suppressor mutations indicates an interaction between the nuclear and mitochondrial genomes. A preliminary biochemical characterization is presented.     

Nuclear suppressors of the mitochondrial mutation oxi1-V25 in Saccharomyces cerevisiae. Genetic analysis of the suppressors: absence of complementation between non-allelic mutants

Ten informational nuclear suppressors of the oxi1- mitochondrial mutation of Saccharomyces cerevisiae are recessive. They are linked to each other, but their allelism is uncertain. Some of them unfavourably affect functions of standard (mit+) mitochondrial genomes. One suppressor severely impairs or entirely prevents mitochondrial functions of the spore clones carrying it. The spectrum of mit- mutations on which these suppressors act is similar to that exhibited by nam3-1. In double heterozygotes namx/NAM3+, NAM+x/nam3-1 the oxi1- (and box3-) mutation is suppressed, yet one of our suppressors (R705) and nam3-1 show independent segregation in tetrads. This indicates that there may be absence of complementation between non-allelic suppressors.     

Genetic basis of mitochondrial function and morphology in Saccharomyces cerevisiae

The understanding of the processes underlying organellar function and inheritance requires the identification and characterization of the molecular components involved. We pursued a genomic approach to define the complements of genes required for respiratory growth and inheritance of mitochondria with normal morphology in yeast. With the systematic screening of a deletion mutant library covering the nonessential genes of Saccharomyces cerevisiae the numbers of genes known to be required for respiratory function and establishment of wild-type-like mitochondrial structure have been more than doubled. In addition to the identification of novel components, the systematic screen revealed unprecedented mitochondrial phenotypes that have never been observed by conventional screens. These data provide a comprehensive picture of the cellular processes and molecular components required for mitochondrial function and structure in a simple eukaryotic cell.     

Genetic analysis of spontaneous and 6-N-hydroxylaminopurine and propiolactone induced Adp+ mutants in Saccharomyces yeasts

652 spontaneous and 6-N-hydroxylaminopurine and propiolactone-induced mutants were obtained in yeast. 598 of them were LYS2 mutants. Detailed genetic analysis of the mutants was performed, including analysis of growth pattern on lysineless medium, suppressibility by nonsense suppressors of three types and localization on the recombination map of the LYS2 gene. Mutants induced by different agents were different for all these criteria, except for distribution among the map regions.     

Null mutants of Saccharomyces cerevisiae Cu,Zn superoxide dismutase: characterization and spontaneous mutation rates.

Deletion-replacement mutations of the Saccharomyces cerevisiae Cu,Zn superoxide dismutase gene were constructed. They were exquisitely sensitive to redox cycling drugs and showed slight sensitivity to other agents. The aerobic spontaneous mutation rate was three- to fourfold higher in sod1 delta 1 mutants, while the anaerobic rate was similar to that of the wild type.     

Pathways of ultraviolet mutability in Saccharomyces cerevisiae. III. Genetic analysis and properties of mutants resitant to ultraviolet-induced forward mutation.

Non-allelic mutants of Saccharomyces cerevisiae with reduced capacity for ultraviolet light (UV)-induced forward mutation from CAN1 to can1 were assigned to seven distinct genetic loci, each with allele designations umr1-1, umr2-1, …, umr7-1 to indicate UV mutation resistance. Each allele complemented rev1-1, rev2-1, and rev3-1. None conferred a great deal of UV sensitivity. When assayed on yeast extract-peptone-dextrose complex growth agar, umr1, umr3, and umr7 (a mating type) were the most UV-sensitive, with a dose-reduction factor of approximately 1.2 at 10% survival. When assayed on synthetic agar lacking arginine, however, umr3 was the most UV-sensitive (dose-reduction factor of 1.5 at 10% survival). UV revertability of his5-2, lys1-1, and ura4-1 was normal in strains carrying the single genes umr4, umr5, umr6 and umr7; umr1 reduced revertibility of his5-2 and ura4-1 but not lys1-1; umr2 reduced only ura4-1 revertibility; umr3 reduced UV reversion of all three test alleles. Five a/α homozygous umr diploids (except umr1 and umr4) failed to sporulate. One of these, umr7, blocked normal secretion of alpha hormone in α segregants and could not conjugate with a strains. The phenotypes of umr mutants are consistent with the existence of branched UV mutation pathways of different specificity, some of which may function in the single RAD6-dependent error-prone pathway for repair of UV damage. Other possible pathways of action are discussed. It is also suggested that regulatory functions interacting with the mating-type locus or its gene products may play some role in UV mutagenesis or error-prone repair.