218 



HOROWITZ AND LEUPOLD 



1.0-1 



0.8- 



0.6- 



0.4- 



0.2- 



Abscissg: Colonies/plate 



Ordinate: Frequency of indispensable functions 



o 



"1 



-r- 



5oo 



loco 



l5oo 



Fig. 3. Frequency of temperatnjre mutants of the indispensable class as a function of 

 population density. Vertical lines show the range within which the experimental 

 points would be expected to fall in 95 per cent of similar experiments. The broken 

 line shows the weighted mean of the distribution. 



quency of indispensable functions, as 

 revealed by the temperature mutant 

 method, is even lower than was indi- 

 cated by the less extensive series of 

 temperature mutants in Neiirospora. 

 Actually, the two results— 46 per cent 

 of indispensable functions in Neiiro- 

 spora, compared to 23 per cent in E. 

 coli—zvc not very different, consider- 

 ing the great differences in the respec- 

 tive organisms. The intensity of the 

 selection which opposes the recovery 

 of multifunctional genes in the usual 

 screening procedures is thus of a rather 

 low order and is incapable of account- 

 ing for the fact that the genes detected 

 by these procedures appear to be pre- 

 ponderantly of the unifunctional type. 



CONCLUSION 



In concluding this paper, we should 

 like to make some brief observations 

 on the significance of the low fre- 

 quency of indispensable functions and 

 the one gene-one enzyme hypothesis. 



Our results indicate that the effect 

 of most lethal mutations in Neiirospora 

 and E. coli is to block the synthesis of 

 metabolites which are replaceable by 

 nutritional means; that is to say, of low 

 molecular weight substances such as 

 might be expected to diffuse into the 

 cell and of which the complete me- 

 dium is chiefly, if not exclusively, com- 

 posed. This situation appears to con- 

 trast markedly with that encountered 



