NONCHROMOSOMAL GENES 



255 



550 



575 



600 



Normal yeast 



^ Petite vegetative (v) 

 (/) 



re 



>■ Petite segregational (s) 

 Zygotefu X s) 



Wild type 



mi — 3 



Poky (mi - 1) 



2 C115 

 o 



Q. 



2 C117 



03 



CI 15 in mj — 3 

 CI 15 in poky 

 C117 in mi — 3 

 CI 17 in poky 



m 



w^ 

 m 





1^ 



M. 



a 



ai 



+ 03 



FIGURE 9.6. Cytochromes of normal and mutant strains of Neurospora and yeast. 

 The absorption bands of the cytochromes are indicated, the wider bands representing 

 higher concentrations. The actual differences are larger than indicated. The positions 

 of the bands of some known cytochromes, c, e, b, a j, and + 03, are shown at the bot- 

 tom of the figure (from Wagner and Mitchell, 1955, Genetics and Metabolism, New 

 York, Wiley, p. 334). 



of the nonchromosomal nature of the petite factor comes from studies of 

 its origin. Ephrussi has shown that the petite mutation can be induced by 

 treating normal yeast cells with euflavin, the most direct demonstration 

 being by means of pedigrees of the sort shown in Figure 9.7. The mode 

 of action of this acridine dye is not known, but its ability to convert 

 a population of normal yeast cells to petites at a concentration which 

 does not kill is a dramatic example of induced mutation, quite distinct 

 from the mutagen-induced chromosomal gene mutations which occur at a 

 frequency of 10"^ or less. 



The change induced by euflavin does not involve the loss of mito- 

 chondria as morphological entities. Both in electron micrographs and in 



