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DISCUSSION 



Mcllzvain: Dr. Magasanik, you explained that the phenomenon of 

 enzyme repression could have some advantage for the cell. I wonder 

 whether you have tackled this experimentally in the sense of putting 

 into competition cells which for instance have, and others which do not 

 have, the mechanism for repressing galactosidase by glucose, etc. This 

 is related to the earlier discussion concerning enzyme activity; and 

 matters such as the K^ of the enzyme, the mutual adjustment between 

 K^ of the enzyme and the level of substrates available to the organism 

 or to the enzyme in the cell, are involved. 



Magasanik : It w ould be very nice to have such mutants but I don't 

 know of any cell which can be induced and is not subject to repression. 

 We believe that even in a cell that is not strongly subject to repression 

 by glucose, there is still some repressive mechanism active, but it is set 

 at a higher level; we would have to obtain mutants that have a very 

 ineffective mechanism as against those with a very good repressive 

 mechanism. 



Pardee: Dr. Novick (personal communication) told me of a mutant 

 which may serve this purpose. Bacteria can be inducible or constitu- 

 tive : inducible bacteria need an inducer to make the enzyme, whereas a 

 constitutive mutant makes the same enzyme at a very high rate all 

 the time without needing an inducer. These bacteria apparently lack 

 the repressor, therefore they do not need an inducer. Novick has com- 

 pared the growth rate of one of these constitutive organisms (which is 

 making about 10 per cent of its dry weight in p-galactosidase) with that 

 of the wild type which is not making this enzyme. The experiment is 

 done in the absence of galactose when there is no function for this 

 enzyme. One organism makes the enzyme, the other does not, and the 

 one that does not make the enzyme grows about 20 per cent faster than 

 the one that does make the enzyme in this case. It is a single-step 

 mutant ; there is probably only one genetical change in the cell. There- 

 fore, there is some possibility that the slower growth-rate is due to the 

 forced formation of a lot of extra protein in this organism. 



Mcllwain : I should like to see the more ecological experiment of the 

 two being put together. 



Pardee : This has been done in what is called a chemostat, something 



