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CHAPTER 32 



of amino acids it contains, their order in the 

 polypeptide, the number of polypeptide 

 chains, the way in which the parts of a poly- 

 peptide chain are arranged relative to each 

 other, and the way in which the different 

 polypeptide chains in a protein are arranged 

 relative to each other. 



It has been possible to study, in some de- 

 tail, the enzyme tryptophan synthetase that 

 is found in the bacterium Escherichia coli. 

 This enzyme can be treated in vitro so that it 

 dissociates into two proteins, i.e., two poly- 

 peptide chains. By itself, neither of these 

 chains has the usual enzymatic activity. But 

 these two proteins can be reassociated, where- 

 upon the normal enzymatic property is 

 restored. Clearly, then, both portions need 

 to be united in order to have this specific 

 enzymatic action, so that part of the enzyme's 

 specificity must be due to the joining of these 

 two polypeptides. (You might suppose that 

 this part of the specificity is due to the primary 

 action of a single cistron.) But this must be 

 only a portion of the total specificity of this 

 enzyme. Another portion of it must reside 

 in the nature of the polypeptide chains, which 

 when joined make not just any enzyme, but 

 tryptophan synthetase in particular. The 

 fact that the two chains are so easily disso- 

 ciable and reassociable indicates that these 

 chains do not have a complex physical rela- 

 tionship when joined together, and suggests 

 that each chain might be specified independ- 

 ently. In this case two distinct cistrons would 

 be involved, each one specifying one poly- 

 peptide chain. 



A number of bacterial mutants were ob- 

 tained which lacked tryptophan synthetase 

 activity.^ Some of these defected one poly- 

 peptide chain and others defected the second. 

 A genetic study showed that all the mutants 

 defecting one chain were recombinationally 

 separable from those defecting the other, 

 although adjacent areas of the genetic map 

 were involved. In this case, then, we have 

 ^ Based upon the work of C. Yanofsky. 



the choice of considering the two adjacent 

 areas either as a single cistron, or as two 

 separate cistrons. Because the detailed 

 specificity of this enzyme seems to depend 

 upon what each of these two genetic areas 

 does individually, we shall consider that two 

 cistrons are involved. On this alternative, 

 each cistron completely specifies a polypep- 

 tide chain, and the union of the two chains 

 comprising tryptophan synthetase may some- 

 how be connected with the fact that the two 

 cistrons involved are adjacent. 



How do these results, and our interpreta- 

 tion of them, affect our general hypothesis of 

 one primary effect-one cistron? The answer 

 is that the general hypothesis is not at all 

 affected. However, the specific hypothesis to 

 test it, one enzyme-one cistron, should be 

 made more comprehensive and be stated as 

 one polypeptide-one cistron. This means that 

 the total specificity of a polypeptide chain is 

 determined by one cistron. According to the 

 general hypothesis, the primary effect which 

 a cistron has would be, at least in some cases, 

 the complete specification of a polypeptide. 



Not all proteins are enzymes. If the hy- 

 pothesis one polypeptide-one cistron is cor- 

 rect, we should predict that each polypeptide 

 chain in all proteins is completely specified 

 by the primary and solitary action of a single 

 cistron. Consider now certain results ob- 

 tained by studying the protein hemoglobin.^ 

 Human hemoglobin has a molecular weight 

 of 66,700. In the horse, and probably in 

 man, too, the molecule is spheroidal in shape, 

 and its dimensions are 55 X 55 X 70 A 

 (Angstrom units). It is composed of two 

 half-molecules which are usually exact dupli- 

 cates of each other. In each half-molecule 

 there are two different polypeptide chains, 

 called a and /3, each containing about 150 

 amino acids. The chains coil to form what 

 are termed right-handed helices, and different 



^ Based upon the work of V. M. Ingram, L. Pauling, 

 H. A. Itano, H. Lehrmann, J. V. Neel, M. F. Perutz, 

 and others. 



