Nuclear factors and the control of suppressiveness in petite mutants of Saccharomyces cerevisiae

Mitotic segregation of cytoplasmic determinants for chloramphenicol resistance in mammalian cells. I: Fusion with mouse cell lines

The segregation of cytoplasmically inherited chloramphenicol (CAP) resistance in mouse cells was investigated in fusions between CAP-resistant cells or cytoplasts (enucleated cells) and CAP-sensitive cells of varying tissue origin. All hybrids formed in cell-cell fusions were initially CAP-resistant, indicating that CAP resistance is dominant. Hybrids from fusions of cells of the same tissue origin (homologous) were stably CAP-resistant, whereas the hybrid population from fusions of different origins (heterologous) showed a rapid diminution of average CAP resistance. Individual hybrid clones from these heterologous fusions also showed an overall loss of CAP resistance, and a wide variation in CAP resistance which is consistent with a large number of genetic determinants (possibly mitochondrial DNA molecules) contributing to the CAP phenotype. Similar results were obtained from cytoplast-cell fusions, so the observed CAP segregation is not the result of nuclear-nuclear interactions. This segregation of CAP resistance constitutes a second criterion of cytoplasmic inheritance in mammalian cells.

A novel class of Saccharomyces cerevisiae mutants specifically UV-sensitive to "petite" induction

A mutant of Saccharomyces cerevisiae has been isolated which, though exhibiting a normal response to nuclear genetic damage by ultraviolet light (UV), is more sensitive than its wild type specifically in the production of the cytoplasmic (rho-) mutation by this agent. Some of the features of this mutation which has been designated uvsrho 5 are: i) The mutation is recessive, it exhibits a Mendelian, and hence presumably nuclear, pattern of segregation, but manifests its effects specifically and pleiotropically on mitochondrial functions. ii) Mutant cells resemble their wild type parents in a) growth characteristics on glucose; b) in their UV induced dose response to lethality or nuclear mutation and c) the ability of their mitochondrial genome, upon mating with appropriate testers, of transmitting and recombining various markers, albeit with enhanced efficiency. Similarly, d) they are able to modulate the expression of mitochondrial mutagenesis by ethidium bromide. Thus their mitochondrial DNA appears genetically as competent as that of the wild type. iii) Mutant cells differ from their wild type parents in a) growth characteristics on glycerol; b) susceptibility to induction of the mitochondrial (rho-) mutation by various mutagens, in that the rate of spontaneous mutation is slightly and that by UV is significantly enhanced, whild that by ethidium bromide is greatly diminished. Conversely, c) modulating influences resulting in the repair of initial damage are diminished fro UV and stimulated in the case of Berenil. iv) The amount of mitochondrial DNA per cell appears elevated in the mutant, relative to wild type, and its rate of degradation subsequent to a mutagenic exposure to either UV or ethidium bromide is diminished. v) A self-consistent scheme to account for this and all other information so far available for the induction and modulation of the (rho-) mutation is presented. In a previous study it was shown that some nuclear mutants of Saccharomyces cerevisiae, more sensitive to lethal damage induced by ultraviolet light (rad) than their parent wild type (RAD), also exhibit a concomitant modification in sensitivity to both nuclear and cytoplasmic genetic damage (Moustacchi, 1971). However, another class of rad mutants respond to the induction of the cytoplasmic "petite" also designated as rho- (or rho-) mutation by UV in a manner indistinguishable from that of the RAD strain. One possible interpretation of this last observation is that some of the steps in the expression of the UV damage on mitochondrial (mt)DNA may be governed by other nuclear and cytoplasmic genetic determinants, the products of which may then act specifically on mitochondrial lesions. If this assumption is correct, it should be possible to find mutants with a normal response to nuclear damage but specifically UV-sensitive towards induction of (rho-)...

Pattern of somatic segregation of the cytoplasmic drug-resistance factors in yeast

The pattern of somatic segregation of the cytoplasmic factors confering resistances to chloramphenicol, erythromycin and oligomyin in S. cerevisiae was studied. The fractions of the zygotes heterozygous for the chloramphenicol-resistance factor and for the erythromycin-resistance factor decreased exponentially with generation number of zygotes. The rate of the segregation was highest for the chloramphenicol-resistance factor and lowest for the oligomycin-resistance factor. The segregation rate as well as the transmission polarity of the chloramphenicol-resistance factor varied with different carbon sources with which the parental haploids were grown prior to mating.

Cytoplasmic inheritance in Saccharomyces cerevisiae: comparison of zygotic mitochondrial inheritance patterns

Mitochondrial movements in Saccharomyces cerevisiae (Sc) zygotes were monitored with phase-contrast microscopy and compared to known mitochondrial inheritance systems. The mitochondria of Sc were convincingly identified by integrated use of phase-contrast, cytochemical and electron microscopic observations. Mitochondria in Sc appear to move by saltatory jumps, which appear to be oriented towards movement of mitochondria into developing buds. Tracking of mitochondria of different genotypes was made possible by positive identification of each mitochondrial population before zygosis, and by the low degree of mixing (less than 10%) of mitochondrial populations before first bud septation. A grande by grande cross demonstrated equal numbers of mitochondria from each haploid moving into the first zygotic bud. A grande by neutral petite cross gave a 2:1 ratio of grande to petite mitochondria. However, a grande by suppressive petite cross gave equal numbers of grande and petite mitochondria. Using drug resistance systems, a comparison was made of highly biased (97%) and moderately biased (71%) chloramphenicol resistant inheritance patterns. In both cases, the ratios of drug resistant to sensitive mitochondria were 1:1. When numbers of mitochondria moving into an individual bud were compared to the phenotypic content of the clone of that bud, no model could be constructed which could predict the latter from the former. The data indicate (with the exception of the neutral petite by grande cross) that the numbers of each mitochondrial type "inserted" into the first zygotic bud are equal, regardless of the degree of asymmetry of inheritance of mitochondrial markers.

The restriction of the recombination of mitochondrial DNA molecules in the zygotes of Saccharomyces cerevisiae

Crosses were made between strains carrying a nuclear control factor (NC) and strains classified with respect to their omega allele (omega+ or omega-). The characteristic asymmetrical transmission was always observed (as was seen) in crosses not involving the omega factor. The analysis of functional recombinants in a cross involving an NC factor has indicated that the absence of the omega effect may be caused by a restriction in the zygote of the recombination of mitochondrial DNA molecules.

Genetic analysis of unequal transmission of the mitochondrial markers in Saccharomyces cerevisiae

The presence of mitochondrial sex factor, omega, was demonstrated in haploid strains of yeast Saccharomyces cerevisiae which came from our laboratory. Transmission and recombination of the mitochondrial genes (CR/CS, ER/ES and OR/OS), conferring the resistance/sensitivity to chloramphenicol, erythromycin and oligomycin, respectively, were non-polar in homosexual crosses and highly polar in heterosexual crosses. Different results were obtained in crosses involving an erythromycin resistant mutant G706E11 (CSEROS) which was found to contain cellular DNA of diploid level. This strain was omega- and showed no alleles from G706E11 (CS, ER and OS) were transmitted to the zygote progeny in preference to the CR, ES and OR alleles. When crossed to omega+ haploid strains, there was a highly polar recombination, but no transmission was seen for the E and O alleles. Polar transmission of markers from omega+ haploid parental strain, characteristic of heterosexual crosses, was noticed only for the C allele. The crosses of G706E11 to omega+ haploids featured an increase in the recombination frequency. The values of % suppressiveness of sigma- petite mutants were relatively low when determined by crossing to G706E11 or to sigma+ diploid strain M2-8C rather than by crossing to sigma+ haploid strains, indicating that there is a positive correlation between the polar transmission of drug resistance markers and the suppressiveness degrees. Genetic mechanism of the anomalous behaviors if mitochondrial genes in crosses involving G706E11 was discussed and interpreted as due to an unbalanced supply of mitochondrial genomes from parental strains.