142 THE BIOSYNTHESIS OF PROTEINS 



(Tavlitzki, 1949; Slonimski and Ephrussi, 1949; Ephrussi and Slonimski, 

 1950; Slonimski, 1949, 1953). 



The mutation responsible for the appearance of respiration deficiency 

 differs from normal mutations in several respects. It occurs spontaneously 

 with a frequency of about 3-10""^ per cell per generation, whereas usual 

 genetic mutations occur one thousand times less frequently. The rate of 

 mutation is as high in diploid as in haploid cells. It can be increased 

 enormously by various treatments: effect of acridine dyes (Ephrussi, 1949, 

 1953; Ephrussi and Hottinguer, 1950), triphenyltetrazolium chloride 

 (Laskowsky, 1954), manganous ions (Nagai and Nagai, 1958), an elevated 

 temperature (Yeas 1956; Sherman, 1959), ultraviolet light (Raut, 1954) 

 and by various other agents (Lindegren, 1959). In the presence of euflavine, 

 under proper conditions, practically every bud formed is a respiration 

 deficient mutant. The mutation affects simultaneously the formation of 

 several enzymes. However, when the yeast cells are treated by a threshold 

 concentration of acriflavine, some of the freshly arisen small colony cells 

 can still give rise to both normal and respiration deficient cells ; it looks as 

 if they remained for a few generations in an unstable state from which they 

 can still revert to the normal type or become irreversibly respiratory 

 deficient (Ephrussi and Hottinguer, 1951). All these features indicate that 

 this mutation is indeed of an exceptional type. It is exceptional also in its 

 hereditary transmission, as we shall see presently. 



Haploid Saccharomyces cerevisiae of opposing mating types can be 

 crossed; the diploid zygotes multiply by budding for unlimited periods, 

 remaining diploid. They can, however, be induced to sporulate by being 

 transferred onto adequate medium. The asci contain four haploid spores. 

 The spores, if isolated, give rise to haploid cultures which will remain 

 indefinitely haploid unless crossed with a haploid of the opposite mating 



type. 



When two haploid yeasts differing by one single mendelian character (A' 

 versus A) are crossed, and the resulting diploid is caused to sporulate, two 

 spores in each ascus contain the allele A, the other two contain the allele 

 A'. This is a regular mendelian segregation of the character. When a normal 

 haploid yeast is crossed with a respiration deficient mutant obtained as 

 indicated above, different results can be obtained, depending on the par- 

 ticular mutant used. Two extreme types have been encountered: the so- 

 called neutral and the suppressive mutants. Crosses of normal yeast with 

 neutral small colony mutant give normally respiring zygotes, and the four 

 spores in each ascus derived from such zygotes are all of the normal 

 respiratory type. Respiration deficiency is not transmitted to any of the 

 spores, no mendelian segregation of the character is observed, the character 

 simply disappears, although in the very same asci typical mendelian 

 markers were shown to segregate in the regular manner. These and other 



