158 CELL HEREDITY 



TABLE 6.1 



Results of Transducing try 0-11 cys B-18 Bacteria 



with Phages Grown on try 6-2 Bacteria. After Similar Samples Were Plated 



on Different Media, They Were Replicated to Other Media 



(From Demerec, 1956, in Mutation, Brookhaven 

 National Laboratory Symposium, p. 75) 



Region of Exchange if Order is: 

 Number ^ try B + + + try B try B + + 



Pheno- of 1:2: 3~rT 1 j 2 ', 3 ', 4 1 ', 2 ', 314 



Medium type Colonies try D + cys B try D cys B + + try D cys B 



next. This hypothesis assumes that the recombinable and functional 

 units coincide. 



THE STRUCTURE OF GENES IN SALMONELLA 



Demerec and his colleagues have made an enlightening analysis of 

 functional units in Salrnunello tijphimurium by the use of transducing 

 phages. A large number of mutants concerned with certain biochemi- 

 cal reactions were obtained; among these, some 450 required histidine 

 and 77, tryptophane. These mutants could be transduced to proto- 

 trophy at a certain frequency when infected by phages grown pre\'iouslv 

 on wild-type Salmonella. Within any group of mutants, individual mem- 

 bers could be transduced by phages grown on another member, but 

 always at lower frequencies than when the phages were grown on wild 

 type. Transduction to prototrophy presumably occurs by recombina- 

 tion between the host chromosome and a fragment of the donor genetic 

 material. Consequently the frequency of wild-type progeny may be a 

 measure of the degree of linkage — the lower the frequency the greater 

 the linkage. All members of a group of mutants can be subgrouped on 



