Section 3 — Molecular and Microbial Genetics 



test strains. The other two spores form homo- 

 thallic cultures which readily diploidize and then 

 are capable of sporulation. The 2: 2 ratios in the 

 asci reflect the segregation of a single gene, D, 

 for diploidization or homothallism. Gene D 

 segregates independently of the mating type locus. 

 Although gene D is seemingly epistatic to the 

 mating type alleles, aandx, it in reality acts by 

 causing their mutation. Under the influence of 

 gene D, either mating type gene mutates to the 

 other allele at rates which range from one mu- 

 tation per two cell divisions to one per thirty cell 

 divisions. The haploid cells fuse as soon as the 

 mutant allele is expressed and the zygotes give 

 rise to stable diploids which are heterozygous for 

 mating type. The heterozygous condition of the 

 mating type alleles blocks any further action of 

 the gene D. 



3.50. Establishment of Respiratory Capacity during 

 Zygote Formation between Complementing 

 Respiratory Deficient Mutants of Yeast. 



H.Jakob and A. SELS(Gif-sur-Yvette, France). 



Two classes of respiratory deficient mutants are 

 known: genie mutants called segregational 

 "petites" and cytoplasmic mutants called vege- 

 tative "petites", both lacking several respiratory 

 enzymes especially cytochrome-oxidase. The 

 cross of two haploid complementing strains (ex. 

 g. cytoplasmic petite genie one, or between two 

 non allelic genie mutants) gives rise to diploid 

 cells able to synthesize the complete respiratory 

 system. 



We have studied the kinetics of the establish- 

 ment of respiratory capacity in such crosses in- 

 volving p5, p7 and p~ neutral mutants, during 

 the zygote formation, and immediately after it. 

 Every cross shows characteristic kinetics which 

 can be followed by the overall respiratory rate 

 and/or by cytochrome-oxidase activity of the 

 extracts. In certain crosses a greatly enhanced 

 respiratory activity is brought about immediately 

 after the zygote formation. 



These experiments will lead to more precise 

 knowledge about the interactions between the 

 different genes or between these latter and the 

 cytoplasmic factor. 



3.51 . The Tetrade Analysis of Some Bakers' Yeasts. 



L. Sedlarova (Bratislava, Czechoslavakia. 



In some hybrids and production strains of the 

 bakers' yeasts a genetic analysis has been made 

 of cell and giant colony shape, fermentation 



characters and the production of biomase. The 

 results of this analysis will be discussed, together 

 with the process of spore copulation. 



3.52. Directed Hereditary Changes of Fermentative 

 Properties of Yeast and Indirect Selection. K. V. 



Kossikov and O. G. Raievskaia (Moscow, 

 U.S.S.R.). 



Mutational change of fermentative properties 

 of yeast Saccluiromyces globosus induced by 

 specific substrate (sucrose) can be markedly 

 enhanced by changing the cultivation conditions. 

 The greatest effect is obtained by increasing the 

 sucrose concentration in the media (up to a 

 certain limit). 



The joint results show that the hereditary 

 changes of fermentative properties in yeast (in 

 the present case the appearance of their ability 

 to ferment sucrose) are associated with their 

 metabolic activity; this fact does not allow to 

 connect these results with the selection of spon- 

 taneous mutations. 



In order to confirm the absence of spontaneous 

 mutations of the character that is being studied 

 in the above-mentioned experiments on directed 

 changes, tests with indirect selection are being 

 carried out. 



The results obtained show the ineffectiveness in 

 the present case of indirect selection. The results 

 will be discussed with due regard of the additional 

 experimental data. 



3.53. Regulatory Mutations Concerning Threonine 

 and Methionine Biosynthetic Enzymes in Sac- 

 charomyces cerevisiae. H. de Robichon- 

 Szulmajster and W. Sly (Gif-sur-Yvette, 

 France). 



In S. cerevisiae, methionine and threonine 

 share a common synthetic pathway which bran- 

 ches from homoserine. The common steps in- 

 clude: 



(A) (B) 



Aspartate v-^ Aspartyl-phosphate ^ Aspartic 



(C) 

 semi-aldehyde «-» Homoserine 



In addition to the control they can exert on 

 enzymes of their own branch (after homoserine), 

 each end product control its individual rate of 

 synthesis, by repression and feedback inhibition 

 of different enzymes in the common pathway. 

 Thus, enzyme A is inhibited and repressed by 

 threonine(l) and enzyme C is inhibited and re- 

 pressed by methionine (2). Structural genes for 

 these two enzymes are unliked(3). 



An attempt has been made to find regulation 



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