GENETIC CONTROL OF CtLL INTEGRATION 347 



This result indicates the segregation of a locahzed site determining 

 receptivity to Dt. It seems likely that other localized sites exist in 

 the a I region which determine response to other controlling elements 

 like Ac and Spm, but there are no further data on this question as yet. 



The prevalence and organized complexity of controlling element be- 

 havior in maize indicate strongly that these systems play a decisive role 

 in normal, nonmutable material. Some hints about this role may come 

 from comparisons with other systems. There are obvious parallels be- 

 tween the controlling elements of maize and those which regulate the 

 rate of enzyme formation in E. coli and Salmonella. One similarity 

 lies in the two-element control system, in which one element acts only 

 in the cis position and is a part of the locus being affected, while the 

 other element can be anywhere in the genome. The possibility that both 

 an intranuclear signal and an "environmental ' one, mediated in the 

 cytoplasm, interact to determine the timing of activation of particular 

 loci is consistent with observations made in both materials. However, 

 the maize material has revealed a far greater complexity of intranuclear 

 interaction than had been anticipated from any other studies, and under- 

 scores dramatically how much is still unknown about gene action. 



Another phenomenon which shows evidence of genetic control of a re- 

 pression mechanism concerns the formation of surface antigens in Sal- 

 monella and in Paramecium. These studies provide another approach 

 to the analysis of genetic control devices, and are of particular impor- 

 tance because the antigens are proteins which can themselves be 

 investigated. 



In Salmonella, the genetic control of phase variation has been 

 analyzed by Lederberg and lino. There are two unlinked loci, H ^ and 

 //.,, which determine the specificity of the flagellar antigens. At any one 

 time, only one of the two potential antigens is present, although the 

 cell carries information for the synthesis of both. With a random fre- 

 quency of 10"'^ to 10~° per cell division, a cell will switch from the 

 antigen determined by one locus to that of the other. This phenomenon 

 is phase variation. The question is: What determines which gene will 

 be expressed and which repressed at any particular moment? 



Employing transduction to study the effects of the phase of the donor 

 and receptor cells upon the phase of the transduced progeny, a clear 

 result was obtained. In transductions of the H j gene, only the phase of 

 the receptor cell was relevant, and no influence of the donor H j gene 

 upon its expression could be detected. The transduced Hg gene, on the 

 contrary, carried information dictating the phase of the transduced 

 progeny as well as their antigenic specificity. When the expressed 

 state of the donor H2 was transduced, all progeny were expressed H2 s 



