Section 3 — Molecular and Microbial Genetics 



mutants, using resistance to amino acid analo- 

 gues for selection. Haploid mutants, resistant to 

 the methionine analogue, ethionine, have been 

 selected. They fall into three classes, recessive, 

 semidominants and dominants. Most of them are 

 methionine (or methionine-precursors) excretors. 



In one of the semi-dominant haploids, in 

 which resistance to ethionine appears to be 

 digenic, aspartokinase (enzyme A) is no longer 

 repressed by exogenous threonine, but regulation 

 of homoserine dehydrogenase (enzyme C) is un- 

 changed. The inhibitibility of aspartokinase is 

 not modified in this mutant. 



These observations were unexpected since 

 ethionine is not an analogue of threonine which 

 is important in the repression of this enzyme. 

 Since non-repressibility by threonine, which re- 

 presents an apparent regulation mutation, was 

 found in one of these mutants, the other members 

 of the three classes of mutants are being examined 

 enzymatically and genetically, in hopes of iden- 

 tifying and mapping the characters which fulfil 

 the criteria of regulation mutations. 



1. De Robichon-Szulmajster and Corrivaux, 

 Biochem. Biophys. Acta (1963). 



2. Karassevitch and de Robichon-Szulmaj- 

 ster, Biochem. Biophys. Acta (1963). 



3. Mortimer and Hawthorne; de Robichon- 

 Szulmajster, unpublished. 



analysed. Different new combinations were 

 found. 



Several other yeasts were used later. 



Using diploid yeasts all the transformed cells 

 were heterozygous for the newly acquired abili- 

 ties. Heterothallic haploid yeasts treated with 

 DNA derived from homothallic diploid yeasts 

 yielded diploid homozygous cells. 



A hypothesis is presented which can explain 

 these facts. 



The mendelian laws still hold, the foreign 

 DNA is localized exclusively in the nucleus. In 

 our opinion the different hereditary characters, 

 dominant or recessive, are transferred independ- 

 ently from each other. 



Attempts to fractionate the DNA preparations 

 by means of an Ecteola column so that only one 

 or a few genes were present in each fraction were 

 unsuccessful. 



Very confusing was a phenomenon which is 

 called pseudo-transformation, a temporary 

 change of characters, which decreases after sub- 

 sequent transferring into the same broth. 



Our previous hypothesis was that the pseudo- 

 transformation was induced by the RNA still 

 present in the DNA preparations. Harris showed 

 that also enzymes still were present. This RNA 

 or the enzymes are adsorbed at the cell walls. 

 Thorough washing, however, did not always 

 remove the pseudo-transformation. It seems that 

 sometimes the substances have reached inside the 

 cells. 



3.54. Transformation in Yeast. W. F. F. Oppenoorth 

 (Delft, The Netherlands). 



DNA has been extracted from bacteria with- 

 out loss of its genetic characters. The transfor- 

 mation of yeast also was possible but to a much 

 less extent. When an experiment was successful 

 only few transformed cells were present. 



The transfer of fermenting abilities was in- 

 vestigated. A yeast, a hybrid obtained by Winge 

 (303-9), not capable of fermenting disaccharides 

 was treated with DNA extracted from Sacch. 

 chevalieri, able to ferment sucrose and raffinose 

 1/3. The DNA was added into malt extract and 

 inocculated with the acceptor yeast. After some 

 days the yeast was full grown and transferred 

 into different sugar Durham tubes. After several 

 transfers cells were isolated with the micro- 

 manipulator and the fermenting abilities were 



3.55. On the Possibility of Carrying Out the Func- 

 tional Test for A I lei ism in the Ascomycete Asco- 

 bolus immersus. Jean Mousseau (Gif-sur- 

 Yvette et, France). 



In some crosses between independent mutants 

 with colourless spores there appear asci con- 

 taining 6 mononuclear spores and 1 binuclear 

 spore. Genetic analysis of such asci shows that 

 in some of them the binuclear spore is hetero- 

 caryotic possessing a nucleus from each parental 

 type. These heterocaryotic spores are of wild 

 type phenotype (brown red colour). This shows 

 that complementation takes place between mu- 

 tations of the parental strains and that the mutant 

 allele is recessive with regard to the wild type 

 allele. These double spores therefore make it 

 possible to carry out a functional test for allelism. 



36 



