COFACTORS OF EnZYMIC ADAPTATION 215 



Induced "petite" mutation 



Ephrussi and his collaborators have shown that clones of 

 normal yeast during their growth constantly give rise to 

 respiration-deficient mutants ("vegetative mutants" or 

 "vegetative petites") stable in vegetative reproduction. The 

 respiratory deficiency was shown to be due to lack of several 

 enzymes (including cytochrome oxidase) firmly bound, in 

 normal yeast, to particles which can be sedimented by 

 centrifugation and to behave as a non-Mendelian character in 

 crosses between normal and mutant yeast. Addition of euflav- 

 ine (2 : 8-diamino-iV-methylacridine) in sufficient concentra- 

 tion induces mutation in almost every newly formed bud, i.e. 

 the mutation rate is close to 1. It was suggested that the 

 mutation consists of a loss, or irreversible functional inactiva- 

 tion, of a particulate cytoplasmic autoreproducing factor, and 

 the question of the possible identity of this genetic material 

 with subcellular units, defined by various biochemical and 

 cytological criteria, was discussed. Vegetative mutants do 

 contain mitochondria that are morphologically similar to 

 those of normal yeast in spite of the fact that they do not 

 contain cytochrome oxidase. References to the various aspects 

 of this work will be found in Ephrussi (1953), Slonimski 

 (1953« and b) and Ephrussi, Slonimski and Yotsuyanagi 

 (1955). 



Our experiments were performed in the following way. To a 

 culture of diploid or haploid yeast exponentially growing in a 

 synthetic, highly buffered medium with excess of glucose, 

 euflavine is added at zero time. After 6-6-5 hours, cells are 

 plated on euflavine-free medium and the number of mutant 

 clones scored. It can be seen from Table II that the difference 

 in growth of the induced and the control culture is so small 

 that the frequency of mutants directly reflects the mutation 

 rate and selection is excluded. Furthermore, for approxi- 

 mately the first half of a cellular generation no mutants appear 

 in the population. The average mutation rate calculated over 

 the period of 2 • 5 generations being • 73, the actual mutation 

 rate is close to 1 for the last two cell generations. 



