Section 4— Gene Action 



not (3-galactosidase for its metabolism, it was 

 possible to select for suppressed mutants of the 

 type z~y+ in addition to z+y+ revertants selected 

 on lactose. The chromosomal location of the 

 suppressors for many of these strains has been 

 determined. They fall into four classes: (1) intra- 

 operon suppressors, and extra-operon suppres- 

 sors which map near (2) the methionine, (3) the 

 serine and (4) the tryptophan markers on the E. 

 coli chromosome. None of the extra-operon 

 suppressors involves translocations of the defect- 

 ive operon to another part of the chromosome. 

 These results are difficult to interpret by the 

 hypothesis that the original mutant is blocked 

 at the level of gene transcription. They are better 

 explained by a model in which the mutation has 

 resulted in an inhibition of translation of mes- 

 senger-RNA into protein. Dominance tests with 

 these suppressors provide further indication of 

 their character. 



4.10. On the Possibility of a Specific Repression at 

 the Genetic Level. M. Luzzati, L. Clavilier 

 and G. Pere (Gif-sur-Yvette, France). 



At present, no case of specific repression of a 

 genetic event is known. Such repression as has 

 been described — i.e. the repression of the form- 

 ation of a set of biosynthetic enzymes by the 

 final product of the pathway — has always been 

 concerned with the phenotype of the cell. 



We have discovered a phenomenon (1 ) which 

 concerns a specific inhibition at a genetic level. 

 We have studied gene conversion of the adl 

 locus in S. cerevisiae. The ad~h mutation leads to 

 a simultaneous requirement for adenine and 

 histidine. 



In a diploid heteroallelic for ad3 the frequency 

 of prototrophic revertants due to gene conversion 

 is of the order of 10 _4 /day/cell. L-histidine(but 

 not adenine) prevents some of 95 per cent of the 

 gene conversion. The conversion of four diffe- 

 rent heteroallelic combinations within the same 

 locus are also inhibited by L-histidine to the same 

 extent. However, L-histidine has no effect on the 

 gene conversion of another locus th2. 



Various reconstitution experiments have per- 

 mitted us to exclude explanations based on a 

 selection mechanism. 



Inhibition is not due to an effect of histidine on 

 the expression of ad3+ gene in a ad3~ cytoplasm, 

 for histidine has no effect on the back mutation 

 of either a haploid or a homoallelic diploid for 

 the same locus. 



If the ad3 gene is a type of operator mutant, 

 one could imagine that the cytoplasmic repressor 

 formed in the presence of histidine might, by 



reacting at the gene level, inhibit gene conversion, 

 which is known to be a mitotic nonreciprocal 

 recombination. 



1. L. Clavilier, M. Luzzati, P. P. Slonimski, 

 Compt. Rend. Soc. Biol. 154, 1970, 1960. 



4.11. Genetic Control of 5-Fluorouracil Resistance in 

 Saccharomyces cerevisiae. Francois Lacroute 

 (Gif-sur-Yvette, France). 



Growth of the wild type is inhibited by a 

 10~ 6 m concentration of 5 FU. Many resistant 

 mutants have been obtained with thresholds 

 varying from 3 X 10~ 5 m to more than 10 _3 m. 



Sixteen independent mutants have been 

 analysed. They show single gene segregation; 

 they belong to four different and unlinked loci 

 and are dominant or semi-dominant. 



Allelic relationships of these 5 FU resistance 

 genes with the four loci for uracil dependence 

 already known in S. cerevisiae and the mecha- 

 nisms of resistance will be discussed. 



The release in the cultures of resistant mutants 

 of a substance promoting the growth of uracil 

 requiring mutants suggests that feed-back in- 

 hibition or repression has been relieved in these 

 resistant strains. 



4.12. Gene Control of Differential Cell Function. 



William K. Baker (Chicago, U.S.A.). 



The pattern of pigmentation in variegated eyes 

 of Drosophila produced by position effect forms 

 a useful system for studying the ontogenetic 

 differentiation of cellular function. A comparison 

 of the size and distribution of twin sectors 

 produced in the eyes by induced somatic crossing 

 over in D. virilis and D. melanogaster with the 

 size and distribution of pigment patterns caused 

 by position effect indicates that the latter patterns 

 also have a cell-lineage basis. 



From the size of certain pigmentation sectors 

 in the variegated eyes, it is concluded that pig- 

 ment potentialities of a given sector of the eye 

 can be determined in early larval development, 

 many days before pigment formation commences 

 in the eye. Evidence is marshalled that this early 

 determination — in spite of the clonal sectors it 

 makes — is not based on any type of somatic 

 mutation in the strict sense. Rather it is a regu- 

 lation of gene action whose level is rather stably 

 inherited within the somatic cells that will 

 comprise one of the sectors of the compound eye. 



40 



