36 



CONTROL MECHANISMS IN CELLULAR PROCESSES 



lowing transfer of genetic material from E. coli; they found that S. 

 dijsenteriae appears to be genotypically i+. 



As indicated above, the i and z loci are part of the Lac region 

 and are thus relatively closely linked. This situation may be con- 

 trasted with the regulatory system of tryptophan synthesis (Cohen 



20r 



(A 



15 



ujlO 



< 



o 



o 



I- 



O 5 

 < 

 -J 

 < 



I 



E 



(O 



Y 



V— 5 o §■ 



,8 



.0 



o 



l&sA 



-d' 



J- 



4 

 HOURS 



6 



8 



Fig. 2-5. /i-Galactosidase formation in heleromerozygotes of Escherichia 

 coli. Matings were performed in the absence of inducer. At times indicated, 

 phage T6 and streptomycin (Sm) were added to all of the cultures, and the 

 inducer, methyi-thio-/i-D-galactoside (TMG), was added to two of the cultures 

 (triangles); the other two (circles) did not receive any inducer. See Pardee 

 ef al. (1959) for the strains, cultivation and mating conditions, assay procedure, 

 and enzyme unit. {Courtesy, A. B. Pardee, F. Jacob, and J. Monod.) 



and Jacob, 1959 ) . In this case, the regulatory locus ( Rtry ) thought 

 to control the formation of an active repressor of enzymes of the 

 tryptophan pathway could be shown by conjugation and transduc- 

 tion experiments to be located in the genome at a considerable dis- 

 tance from (presumably structural-tvpe) determinants governing 

 the synthesis of these enzymes. Mutations at this regulatory locus 



