22 THE BIOSYNTHESIS OF PROTEINS 



final shape of a protein is not completely determined by the corresponding 

 locus and that factors which are independent of this locus may play a part 

 in shaping the protein molecule. This calls for the same remarks as the cases 

 of complementation between non-identical alleles (cf. p. 20). The genetic 

 locus provides information — perhaps all the information — concerning the 

 linear arrangement of the amino acids in the polypeptide chain (the so- 

 called 'primary structure of the protein'). But the activity of an enzyme or 

 the immunological properties of a protein depend on their tertiary struc- 

 ture. The folding and association of polypeptide chains are conditioned by 

 the amino acid sequence, but they might also depend on contingent factors 

 like the concentration of various ions and possibly on somewhat more 

 specific actions. Certain suppressors might act by modifying such factors. 



(c) Controlling genetic units. Another example of complication to the 

 gene-enzyme relationship might be seen in the genetic control of ^-galacto- 

 sidase synthesis in Escherichia coli. The production of the enzyme is con- 

 trolled by three independent and separable loci z, i, y. Only one of those, 

 the z locus, provides structural information for /S-galactosidase synthesis. 

 The / locus controls the actual production of the enzyme, it is involved in 

 the regulation of the synthesis. This locus does not carry any information 

 as to the structure of ^-galactosidase since mutation of this gene does not 

 alter the structure of the protein produced. The third gene y also regulates 

 the synthesis of ^-galactosidase indirectly, by controlling the formation of 

 a system which concentrates into the bacteria inducers of jS-galactosidase 

 synthesis (Jacob and Monod, 1959). A similar situation exists for tyrosinase 

 in Neurospora (Horowitz et al., 1960) and most probably for other inducible 

 and repressible enzymes (see Chapter V). 



Actually, these facts are in perfect agreement with the concept that all 

 or at least an essential part of the structural information relevant to a 

 specified protein is located in a restricted part of the genome, in the present 

 case the z locus. 



(d) Concluding remark. In discussing the value of the one gene-one 

 enzyme hypothesis, it should be clearly realized that two difi^erent mean- 

 ings can be attributed to this relation depending on the direction in which 

 it is expressed. This relation can be taken to mean that the structure of a 

 protein is specifically controlled by a restricted section of genetic material; 

 we have seen that this statement is supported by numerous experimental 

 data and contradicted by none, if we reserve a few cases that have not been 

 thoroughly analysed yet. 



The other acception of the gene-enzyme relationship concerns the mode 

 of action of the genetic material ; this side of the matter has not been dis- 

 cussed here. It should be made clear that there are reasons to doubt that 

 the genetic material exerts its action on the cell exclusively by controlling 

 the structure of specific proteins. The genetic control of differentiation in 



