338 CELL HEREDITY 



Support for this interpretation comes from further studies of cross 1, 

 undertaken to elucidate the lag period before repressor formation begins. 

 It was observed that blocking of protein synthesis by chloramphenicol 

 or by 5-methyltryptophane did not change the rate of repressor forma- 

 tion, strongly indicating that the repressor is not a protein, or a product 

 of protein synthesis in the zygote. It is suggested that the repressor 

 may be a polynucleotide synthesized on the Hfr DNA after its entrance 

 into the F~ cell. 



An important property of the i gene product Ri is its capacity to deter- 

 mine the inducibility both of y and of z. As can be seen from Table 

 11.3, R is able to influence y"*" and z^ independently, regardless of the 

 state of the other factor. Furthermore, y and z need not be on the same 

 chromosome as t for R to exert its effect. Crosses of the type: Hfr z"*" i~ 

 X F~ z~ i'^ give rise to zygotes which require inducer for the formation of the 

 enzyme. Thus, the i"*" factor on one chromosome influences the action of 

 z^ on the other chromosome. This evidence does not clarify whether R 

 acts at the chromosome level or in the cytoplasm (see p. 331). 



Another class of mutants has been described which behave quite dif- 

 ferently. They are the o mutants, which also map in the lac region. 

 Mutants designated o° appear to have neither |3-galactosidase nor per- 

 mease, nor repressor. They could be considered deletions for the entire 

 region were it not for the fact that they are revertible to the wild-type 

 state, in which all three activities reappear. Mutants designated o'^ 

 determine the constitutive synthesis of both enzymes, even in the pres- 

 ence of i^. However, the o'^ element must be on the same chromosome 

 as the factors which it influences. The simplest view of these findings 

 is that repression and induction, under the influence of o, occur at the 

 chromosome level, not at the site of enzyme formation. If this assump- 

 tion is correct, the o gene exerts what might be called a position effect. 



There are hints of similar phenomena in the literature of microbial 

 genetics, e.g., at the gal locus in E. coli and in the histidine region in Sal- 

 monella, but none of them have yet been reported in detail. For a better 

 insight into the possible significance and generality of these findings, it 

 is necessary to consider these results in relation to other information 

 about position effect. 



POSITION EFFECT AND CONTROLLING ELEMENTS 



We define position effect as the influence of one chromosomal region 

 upon the expression of a neighboring region on the same chromosome. 

 The first evidence of this totally unexpected phenomenon came from the 



