Induction of meiotic chromosomal malsegregation in yeast.

A system to detect chromosome number abnormalities occurring during meiosis in Saccharomyces cerevisiae is described. It is based on selection of spores carrying 2 multi-marked chromosomes V. Each step of the technical procedure is critically analyzed and the origin of some biases discussed. Selection and subsequent genetic analysis allow the estimation of the frequency of spontaneous and induced diploid and aneuploid n + 1 (diplo-V) spores. Data are reported concerning the effect of 53 chemical compounds. The great majority of active chemicals induce diploid clones while a minority cause non-disjunction of chromosome V.

Disomic and diploid meiotic products induced in Saccharomyces cerevisiae by the salts of 27 elements.

The effects of salts of 27 elements on recombination and on the production of disomic and/or diploid spores during meiosis of Saccharomyces cerevisiae has been investigated. Be(NO3)2, MgSO4, FeSO4, CuSO4, AgNO3, Na2HAsO4 were inactive on the events studied during meiotic cell division. AuCl4, CdCl2, C4H6O4Pb, SnCl2, K2Cr2O7, RbCl induced both disomic and diploid spores. LiCl acted similarly and also affected recombination. Activity in the induction of disomic spores was shown by MnSO4, HgCl2 and SrCl2. CsCl, CaCl2, Na2MoO4, NiCl2, K2PtCl4 increased the frequency of diploid spores, while NaWO4, VOSO4, KCl, BaCl2 already increased recombination frequency. NaBiO3 showed an effect on meiotic recombination only. A decrease in the occurrence of both diploid and disomic spores was suggested by the data obtained with CoCl2.

Meiotic Diploid Progeny and Meiotic Nondisjunction in SACCHAROMYCES CEREVISIAE.

Abnormalities in chromosome number that occurred during meiosis were evaluated with a specially-constructed diploid strain of Saccharomyces cerevisiae. The strain is heterozygous for six markers of the right arm of chromosome V and heterozygous for cyh2 (resistance to cycloheximide) on chromosome VII.-Selection of meiotic spores on a medium containing cycloheximide and required nutrilites-except those for the markers of the right arm of chromosome V-allows the growth of aberrant clones belonging only to two classes: a) diploid clones, caused by failure of the second meiotic division, with a frequency of 0.54 x 10(-4) per viable spore; and b) diplo V, aneuploids derived from nondisjunctions in meiosis I or meiosis II, with a total spontaneous frequency of 0.95 x 10(-4) per viable spore. About two-thirds of the aneuploids originated during meiosis I, the rest during meiosis II. An investigation of these events in control meioses and after treatment with MMS, Benomyl and Amphotericin B suggests that this assay system is suitable for screening environmental mutagens for their effects on meiotic segregation.

Mutants resistant to manganese in Saccharomyces cerevisiae.

Several mutants resistant to Mn(2+) have been isolated and characterized in Saccharomyces cerevisiae. All the mutations are semidominant and allelic to a single nuclear gene (MNRI). Mg(2+) in the growth medium reverses the inhibitory effect of Mn(2+) in a competitive way. This appears to be due to the inhibition of the uptake of Mn(2+) by the cells, not to an increase of the amount of Mg(2+) inside the cells.The analysis of the distribution of Mn(2+) taken up by growing cells shows that the amount of the ion present in insoluble form is far higher in resistant than in sensitive cells. We therefore believe that yeast cells have a sequestering system for Mn(2+) and that the major difference between mutants and wild-type strains lies in the much higher efficiency of this system.

Genetics and biochemistry of resistance to axenomycin in Saccharomyces cerevisiae.

Axenomycin inhibits protein synthesis in vivo and in vitro in Saccharomyces cerevisiae. The antibiotic acts by binding to ribosomes, most probably to the large ribosomal subunit. Mutant strains resistant to axenomycin appear to contain ribosomes that are not inhibited by the antibiotic. The responsible gene has been mapped on the VII chromosome between the centromere and the leu1 gene.

Effect of X-irradiation on frameshift and missense mutations in Saccharomyces cerevisiae.

In cell populations of Saccharomyces cerevisiae homogeneous for sensitivity to X-irradiation, induction of base insertions/deletions and base substitutions was quantitatively analyzed in a reversion system. The repair mechanisms phenotypically unexpressed in the sensitive cell fraction and fully operating in resistant cells did not affect point mutations of either type.

Effect of mutation in the aromatic amino acid pathway on sporulation of Saccharomyces cerevisiae.

Mutations in ARO1 and ARO2 genes coding for enzymes involved in the common part of the aromatic amino acid pathway completely block the sporulation of Saccharomyces cerevisiae when in a homozygous state, whereas mutations in all the other genes of the same pathway do not. This effect is not due to the lack of any intermediate metabolite but rather to the accumulation of a metabolite preceding chorismic acid. Shikimic acid or one of its precursors was identified as the possible inhibitor. The presence of the three aromatic amino acids in the sporulation medium restores the ability to undergo meiosis. This seems not to be due to a feedback inhibition of the first enzymes of the pathway but rather to a competition between aromatic amino acids and the inhibitor on a site specific for the meiotic process. The inhibition of sporulation seems to occur at a very early step in meiosis, as indicated by the lack of premeiotic DNA synthesis in aro1 and aro2 mutants.

Molecular specificity of x-radiation and its repair in Saccharomyces cerevisiae.

Molecular specificity of soft X-radiation has been studied in yeast by analyzing the transitions UAA in equilibrium UAG and nonsense leads to sense mutations in the codon tyr7-1. Synchronized cell populations in the most radiosensitive and radioresistant stages were compared: they did not show any qualitative or quantitative differences in their sensitivities to the mutagenic action of X-rays. We conclude that repair mechanisms, which remain unexpressed in the sensitive cells, do not affect point mutations of the base-substitution type.