72 Information Storage and Neural Control 



DISCUSSION OF CHAPTER IV 



Mike McGlothlen (Houston, Texas) : What about suppressor 

 genes where you have a mutation of the structural gene and then 

 a counter-mutation of the type that causes the still mutated 

 structural gene to produce normal enzymes? 



Harrison Echols (Madison, Wisconsin): The theory which is 

 now usually advanced to explain these suppressor mutations is 

 that a suppressor is a mutation which has affected the translation 

 mechanism; i.e., it has perhaps affected the ability of the soluble 

 RNA to bind the correct amino acid. The soluble RNA then makes 

 mistakes which partially rectify the mutational mistake. For 

 example, suppose that the original change in the protein was a 

 substitution of the amino acid alanine for glycine and that the 

 suppressor mutation is such that some of the time, in protein 

 synthesis, glycine is put back in place of alanine. In this case, 

 you would now get a reduced level of the original premutation 

 type of protein. Certain suppressors may also involve a change in 

 the concentration of some cell constituents, which leads to the 

 activation of a mutationally altered protein. 



McGlothlen: Would you care to say anything about the origin 

 of the secondary and tertiary structure of proteins? Presumably, 

 the sequence of amino acids is controlled by sequences in DNA, 

 but what about the folding, etc., that produces the active un- 

 denatured form of an enzyme? 



Echols: We think that this comes about purely from a deter- 

 mination of the primary structure. The secondary structure is a 

 matter of solution thermodynamics. A repeating chain of amino 

 acids forms an alpha helix if the solvent is not too hard on hydrogen 

 bonds. To get the specific three-dimensional structure is a tougher 

 problem. However, we can imagine that as the newly synthesized 

 protein comes off the ribosome, there are regions of the protein 

 which are capable of bonding and are in very close proximity to 

 each other. There is actually some evidence that the primary 

 structure does determine the three-dimensional structure of the 

 protein ribonuclease. This derives from experiments in which the 

 protein is unfolded and then caused to fold again. One can break 

 all four of the disulfide bonds by reduction to SH, and unwind 

 the protein into a completely random coil. One would expect, 



