180 Discussion 



DISCUSSION 



Pontecorvo: I suggest we add another group of organisms, namely the 

 filamentous fungi. You mentioned dominance and recessiveness. In 

 Aspergillus we can use classical techniques to identify and locate genes. 

 Roper has an acraflavine-resistant mutant at one locus which is partially 

 dominant in the heterozygote, i.e. the heterozygote is intermediate in 

 resistance. Then he has, at a different locus, a completely recessive 

 mutant resistant to the same drug. If permeability is responsible in 

 these two cases, as you pointed out there must be quite a number of 

 different ways of altering it. 



Davis: I am interested to learn of this work. I wonder whether the 

 mutation to resistance at one of these loci might not involve altered 

 permeability while that at the other locus caused resistance through 

 quite a different mechanism. 



Pontecorvo : But the question of recessiveness that you raised is more 

 likely to fall into this type of category. 



Eagle: There are two systems in which there is an indication that 

 differences in resistance reflect an alteration in a cell component, rather 

 than differences in permeability. One is the recent demonstration by 

 Saz that Esch. coli contains a nitro reductase which is strongly inhibited 

 by aureomycin, and which contains both a protein and flavin moiety. 

 In the nitro reductase of the parent sensitive cell, the flavin dissociates 

 readily; while in the enzyme complex deriving from aureomycin- 

 resistant cells the flavin and protein have a greatly increased affinity. 

 The relationship of this difference to aureomycin resistance, specifically, 

 is not entirely clear; but what seems to be quite clear is that in the resis- 

 tant cell there is a nitro reductase which is no longer inhibited by aureo- 

 mycin, and the flavin component of which is strongly bound to the 

 protein. The development of resistance is associated with a specific and 

 qualitative change in an enzyme normally vulnerable to aureomycin. 



The other system relates to penicillin resistance. Bacterial species as 

 they occur in nature differ markedly in their binding affinity for peni- 

 cillin, and in direct proportion to their penicillin-sensitivity. The sensi- 

 tive organism is one which has a high binding affinity for penicillin, so 

 that lethal concentrations are attained in the cell with a relatively low 

 concentration in the environment. That lethal cellular concentration 

 was of the same order of magnitude for all bacteria examined. Here 

 again we are not dealing with a permeability difference; for the same 

 difference in penicillin-binding capacity between sensitive and resistant 

 organisms is evident in cell-free sonates. Macromolecular components of 

 those sonates differ, as do the whole cells, in their affinity for the drug. 

 Now the curious thing is that this relationship between binding affinity 

 and penicillin sensitivity applies only to strains as they occur in nature. 

 If one takes a sensitive strain and makes it resistant by appropriate 

 selection, that development of resistance is not associated with changes 



