14 THE BIOSYNTHESIS OF PROTEINS 



thus possible to determine rapidly the recombination frequency of series 

 of mutations, and to study quantitatively recombinations which occur with 

 a very low frequency, that is — according to the basic assumption — 

 recombinations between closely located sites. 



When several mutants all lacking the same enzyme activity were isolated 

 and mapped, it was found that all the mutations are located within a narrow 

 region of the genome. This is the genetic region corresponding to the 

 enzyme in question. Crosses between these various mutants, however, may 

 give rise — with a very low frequency — to wild type organisms which 

 produce the normal enzyme, as if recombination had taken place between 

 different mutation sites within the genetic region corresponding to the 

 enzyme (locus). 



Mapping of several such mutants on the basis of the frequency of 

 recombination again led to their arrangement in a linear order, this time 

 within the locus of a single enzyme. Although deviations from the pro- 

 portionality of frequency with distance are often observed for very short 

 distances, the linear order is maintained. The linear arrangement of the 

 mutation sites within a locus found strong confirmation in a recent analysis 

 of the topological relationships between markers in a bacteriophage, which 

 was based on a completely different principle (Benzer, 1959). 



Each mutation thus corresponds to an alteration at one of many possible 

 sites within the linear array which makes up the genetic region correspond- 

 ing to a given enzyme protein (Pontecorvo, 1958; Demerec, 1956; Demerec 

 et aL, 1958; Hartman, 1958). The linear structure of this locus is of funda- 

 mental significance for the mechanism of protein synthesis. 



Recombination after transduction in Salmonella showed that two 

 mutation sites corresponding to two different enzymes may not be more 

 distant from one another than two mutation sites within the same locus. 

 This indicates that two loci can be very close to one another, and most 

 probably contiguous (Hartman, 1957; Demerec et aL, 1958). 



The elementary units of heredity cannot be visualized any more as well 

 separated beads on a string; they are part of a much more continuous 

 structure. It is obvious that the use of the term 'gene' to designate any 

 specific piece of genetic material would now lead to confusion, and there 

 has been a great deal of apparent conflict during the last few years, due to 

 the fact that the same words were being used with different meanings by 

 different authors. Benzer (1957) has greatly helped to clarify the field by 

 introducing new terms for the elementary units of heredity: 



The muton is the smallest unit the alteration of which results in a muta- 

 tion. There is space for many mutons within the locus of an enzyme. The 

 muton is of great interest for protein synthesis, for it is directly related to 

 the smallest modification of the genetic material which causes a change in 

 the structure of the protein produced. It will be very interesting to estimate 



