Genetic Control of Protein Synthesis 73 



just by considering random re-formation of four disulfide bonds, 

 tliat 105 possible alternative forms of the protein should exist. 

 However, what is found is that oxidation to bring back the disulfide 

 bonds produces something like 90 per cent enzymatically active 

 protein. So even though the system is far from physiological, with 

 this protein, at least, one appears to get the total three-dimensional 

 structure purely from the primary structure. 



Arthur Shapiro (New York, New York): A protein is, of 

 course, made up primarily of amino acid chains; but most pro- 

 teins, particularly the specific ones— enzyme proteins — do contain 

 polysaccharides and do contain very specific, important, and 

 critical terminal groups. Is it the general notion now that the 

 DNA chain contains the information that determines these non- 

 amino acid fractions in the protein molecule as well, or is this 

 supposed to be a different kind of thing? If so, is there any clue 

 as to what? 



Echols; In general, as proteins and enzymes have been purified 

 more carefully and more successfully, they have been found in 

 most cases to contain nothing except amino acids. I would feel 

 that the appearance of a sugar group or some other moiety attached 

 to a protein would either be a nonspecific accident or the result 

 of a specific site built into the amino-acid-determined structure 

 of the protein. In other words, I think the complete specificity 

 of the protein comes about because the DNA specifies the amino 

 acid sequence. 



Frank Morrell (Palo Alto, California): We know of some 

 agents that can alter DNA, such as x-ray, etc. Could you elaborate 

 on the sorts of agents that can selectively alter base sequence in 

 RNA? 



Echols: One which is widely used is nitrous acid. This removes 

 the amino groups from bases, producing a change of the base 

 cytosine into the base uracil in RNA. 



Morrell: What is the consequence of alteration of the base 

 sequence in RNA without simultaneous alteration of DNA? 



Echols: Mutagenic agents which aff'ect RNA but not DNA are 

 not known. Thus, in treating a bacterium with a mutagenic 

 chemical, both the DNA and the RNA are involved. To my 

 knowledge, no one has been able to purify a messenger RNA. 



