20 THE BIOSYNTHESIS OF PROTEINS 



Cases of complementation should be analysed very closely before being 

 taken as evidence for the resolution of the genetic locus of an enzyme into 

 several cistrons. The notion of cistron would recover its full value in such 

 cases if the 'function' considered was the production of a certain arrange- 

 ment of amino acids, but it is not an easy matter to study such a function. 



The conclusions from the data examined in the preceding pages can be 

 summarized as follows : the genetic information which controls specifically 

 the arrangements of the amino acids of an enzyme is contained in a unique 

 segment of genetic material, the locus of the enzyme. This locus usually 

 acts as a unit; it may, in some cases, be composed of two or a quite small 

 number of functional units. 



3. On a few Complications on the One Gene-One Protein Relation 



The fact that two contiguous pieces of genetic material may be involved 

 in the control of the structure of one single enzyme does not contradict the 

 one gene-one protein hypothesis, provided one is ready to readjust the 

 definitions of gene and enzyme adequately. But a few categories of 

 experimental data seem to conflict with basic ideas contained in the 

 hypothesis. 



(a) Pleiotropy. In higher organisms, it is not exceptional that a single 

 mutation changes several morphological or physiological characters. This 

 has little impact on the hypothesis for it is highly probable that one single 

 biochemical deficiency in the zygote will have several eflFects in the course 

 of development and may in the end change the properties of the differenti- 

 ated cells in various ways. But pleiotropy has been observed also for bio- 

 chemical characters in micro-organisms. A single mutation can result in a 

 double requirement. 



In well analysed cases, however, the double requirement was shown to 

 result from the lack of one single enzyme. For instance, the double require- 

 ment for methionine and threonine in a Neurospora strain results from a 

 single genetic block in the pathway of synthesis of homoserine, which is a 

 common precursor of threonine and methionine (Teas et al., 1948; Horo- 

 witz, 1956). The double requirement for valine and isoleucine is due to the 

 inhibition by a precursor of isoleucine of the transaminase which makes 

 valine. The precursor accumulates when the transaminase which makes 

 isoleucine is absent (Bonner, 1946). A mutant lacking the latter enzyme 

 therefore requires both valine and isoleucine for growth. A comparable case 

 has also been described for Escherichia coli (Myers and Adelberg, 1954; 

 Rudman and Meister, 1953). 



(b) Suppression. A more puzzling phenomenon is the suppression of 

 a nutritional requirement by a mutation which occurs in a region of the 

 genome completely unrelated to the locus in which the first mutation has 

 occurred. 



