18 THE BIOSYNTHESIS OF PROTEINS 



of a hybrid enzyme in addition to the two parental types in a maize hetero- 

 zygote is most probably another example of the same phenomenon 

 (Schwartz, 1960). 



To summarize, genetic evidence so far indicates that the information 

 necessary for the synthesis of a protein molecule is contained in one or 

 possibly in a very small number of cistrons. 



It should be realized that the analysis has reached a point where the 

 definition of the genetic unit of function, that of protein molecule and that 

 of enzyme are at stake or raise difficulties. This is illustrated by recent 

 studies on tryptophan synthetase in Escherichia coli (Lerner and Yanofsky, 

 1957; Crawford and Yanofsky, 1958; Yanofsky and Stadler, 1958; 

 Yanofsky, 1959). 



The normal enzyme as purified and isolated from the wild strain catalyses 

 three different reactions: 



indole glycerophosphate > indole +triose phosphate (1) 



indole + serine >■ tryptophan (2) 



indole glycerophosphate + serine >■ tryptophan + triose phosphate (3) 



Some mutants lose activity (1) and (2) but keep the other catalytic activity 

 (3). On the other hand, the mutants affecting tryptophan synthetase belong 

 to two contiguous pieces of genetic material. The normal protein trypto- 

 phan synthetase can be split into two inactive moieties which recombine in 

 vitro into a complex having all the activities of tryptophan synthetase. 

 Thus an enzyme having several related but clearly distinguishable catalytic 

 properties results in the present case from the association of two protein 

 structures the formation of which is controlled by two contiguous genetic 

 regions. 



Going one step further in this direction, we will observe, as in Salmonella, 

 clusters of contiguous genetic loci arranged in the same relative order as the 

 steps in the pathway of biosynthesis they control (Demerec and Hartman, 

 1959; Yanofsky and Lennox, 1959; Hartman et al., 1960). 



The names we give to pieces of genetic material and what we call 

 enzymes or an enzyme system is not very important. The fundamental facts 

 are that the smallest piece of protein material which can be made indepen- 

 dently is of the size of a protein molecule or of a large polypeptide chain, and 

 that all the specific information concerning such a major protein piece is 

 found within a unique and limited region of the genome.* 



* An objection can be opposed to this conclusion (Atwood and Mukai, 1953). If 

 many proteins should share a common intermediate, for instance a quite small 

 peptide, a mutation which would prevent the synthesis of this small intermediate 

 would almost certainly be lethal and the mutant could not be detected. A selection 

 is therefore operated and the mutants recovered necessarily correspond to genes 

 controlling a restricted function such as the formation of one elaborate piece of 

 macromolecule. This is a purely theoretical objection, which it is important to keep 

 in mind, but there is no positive evidence that this situation exists. 



