650 CHAIRMAN: J. REINER 
the enzyme but this has not as yet convincingly established whether the loss of activity is accom- 
panied by the degradation of the apoenzyme. The same question would remain for the proteolytic 
enzymes. 
Horowitz: Dr. Cowre, did I understand you to say that Candida is maximally repressed? The 
reason I ask is that I have almost arrived at the conclusion that feedback repression is shown 
only in bacteria, and if there is evidence that it occurs also in yeast, this would be very interesting. 
Cowte: If I did, I made a mistake. I am only talking about inhibition. There are two pro- 
cesses one has to consider; one is the synthesis of an enzyme, the other the activity of the 
enzyme per se. My remarks are restricted to the activity of the enzymic system. It is postulated 
that there is sufficient material provided by the endogenous flow of carbon to keep the internal 
pool, which is fixed in size, almost completely saturated during exponential growth. This material 
as well as exogenous amino acids may compete to form the internal pool by complexing with 
protein (and nucleic acid). This complex prevents any further competition until protein is formed 
and these sites again become available for competitive selection. 
The kind of negative feedback reactions observed in vityo takes place in systems which have 
lost the internal organization of the living cell. In such cellular extracts one can introduce materials 
which inhibit specific enzyme reactions. In the living cell the system appears to remain saturated 
and complexed until proteins are made, and inhibition by end-product amino acids cannot occur 
as long as the complex sites are filled. 
Horowitz: Do you know of any evidence in yeast that indicates derepression of the kind 
that has been found in bacterial mutants? 
CowleE: Not to my knowledge. 
Hatvorson: We have performed experiments designed to test over-all repression in yeast. 
Two cultures of yeast cells were grown, the first on synthetic media and the second on a rich 
mixture of amino acids, vitamins and nucleic acid precursors. The proteins of both were extracted 
and fractionated on DEAE cellulose columns. We expected to find a significant difference between 
the two extracts as a result of repression of the biosynthesis enzymes. To our surprise the protein 
fractions were completely identical. 
Eacte: In animal cells also we have seen no example of negative feedback inhibition in the 
biosynthesis of serine, glycine, homocystine or proline. Large amounts of exogenous amino acid 
have no effect on neosynthesis from the appropriate precursor. 
