410 



(HAPTI.K 32 



acid, serine. h\ the enzyme, tryptophan syn- 

 thetase. Separately occurring tryptophan- 

 requiring point mutants arc obtained which 

 are blocked in the final synthetic step. All 

 oi 25 mutants qualifying prove to be located 

 on the same chromosome and at about the 

 same locus. The second test involves the 

 final step in the synthesis of adenine, cata- 

 lyzed by the enzyme, adenyhsuccinase, 

 which removes succinic acid from adenylo- 

 succinic acid to leave adenine. Of 137 in- 

 dependently occurring point mutations with 

 little or no adenylosuccinase activity, all 

 prove again to be on the same chromosome 

 and at about the same locus. The genes 

 specifying different enzymes are different, 

 each occupying separate loci in the genome. 

 These results and similar ones for other 

 enzymes in Neurospora offer strong support 

 for the hypothesis that the catalytic ability 

 of all enzymes is under gene control. More- 

 over, the addition of B,, tryptophan, or 

 adenine to the diet of mutants defective in 

 the enzymes directly responsible for their 

 respective syntheses makes the mold com- 

 pletely or almost completely normal, pro- 

 viding good evidence that the genes in- 

 volved have only one function to perform 

 — determining the catalytic ability of one 

 enzyme. If a gene had more than one 

 primary effect, nutritionally overcoming one 

 defect would not be expected to produce 

 normality or near-normality in all cases. 

 Because in all these cases the enzymatic de- 

 fect is due to a defect only in one specific, 

 localized area of the genetic map, the total 

 catalytic ability of an enzyme seems to be 

 the result of the primary action of a single 

 gene. 



One Gene-One Polypeptide Hypothesis 



All enzymes are protein, at least in part, and 

 the catalytic ability of an enzyme is known 

 to be due to its protein content and often 

 added co-factors. Proteins are composed 

 of amino acids (Figure 32-4) linked to each 



other by peptide bonds between carboxyl 

 and amino groups to form polypeptide 

 chains. The catalytic ability of an enzyme 

 depends upon the number and kinds of 

 amino acids contained, 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. 



The enzyme, tryptophan synthetase, found 

 in Escherichia coli, can be treated in vitro 

 so that it dissociates into two proteins, that 

 is, two polypeptide chains. Neither single 

 chain has the usual enzymatic activity but, 

 when the two chains are reassociated, nor- 

 mal enzymatic activity is restored. Clearly, 

 to have the specific enzymatic action both 

 chains need to be joined. Since the two 

 chains are so easily dissociable and reas- 

 sociable, probably no complex gene-directed 

 physical or chemical change is needed to 

 join them together. Therefore, the basis for 

 the catalyzing ability of the enzyme must 

 reside primarily in the nature of the poly- 

 peptide chains which, when joined, make not 

 just any enzyme, but tryptophan synthetase 

 in particular. This reasoning leads to the 

 suggestion that each chain might be the re- 

 sult of the primary action of a different gene. 



A number of bacterial mutants lacking 

 tryptophan synthetase activity can be ob- 

 tained. 1 Some of them are defective in one 

 polypeptide chain, and others are defective 

 in the second. All the mutants causing de- 

 fects in one chain are found to be recom- 

 binationally separable from those producing 

 defects in the other, although adjacent areas 

 of the genetic map are involved. In this 

 case we have the choice of considering the 

 two adjacent areas either as a single func- 

 tional gene or as two separate genes. Be- 

 cause the nature of this enzyme seems to 



:; Based upon the work of C. Yanofsky. 



