104 MOLECULES, VIRUSES, AND BACTERIA 



phage T6 and to streptomycin, made /8-galactosidase only when an in- 

 ducer was present, and could not form galactoside permease. In sym- 

 bols, this cross is represented as: 



Hfr ( Pr+ T6^ St^ i- z+ y+ H- ) X F- ( Pr- T6R St^ i+ z+ y- H+ ) 



It was planned to see if H behaved as a single gene character, and, if 

 so, whether it was linked with any of the other markers. Recombinants 

 were selected on the basis of being Pr+ and St^ and were scored for 

 the other characters. Of 80 Pr+ oflFspring tested, 74 were found to have 

 high levels (H~ ) of yQ-galactosidase and six were found to have normal 

 levels (H+ ). Of the 74 H~ bacteria, 73 were found to be constitutive 

 (i") and one inducible (i+). These results show that the genetic 

 alteration leading to high levels of enzyme production behaves as if it 

 were a genetic change closely associated with the region near the Pr 

 locus, probably because it occurs in the lac region. Moreover, this 

 alteration is apparently not in the i gene. 



Proof of closer association of the H character with the lac region 

 than that shown by the crosses just described comes from experiments 

 on the genetic transduction of this character being performed by 

 T. Horiuchi. He grows phage Pike on strains (H~ z+ y+) which pro- 

 duce high levels of /?-galactosidase, and he uses the phage produced to 

 transduce a lac-minus strain (H+ z+ y~) to a lac-positive strain. Al- 

 though it is difficult to isolate relatively rare lac-positive organisms 

 when they are H" and thus inhibited by lactose, Horiuchi has suc- 

 ceeded; he finds that a large fraction of the lac-positive transductants 

 do make /3-galactosidase at a high rate. Thus, the H character is de- 

 termined by the state of some part of the lac region; but, so far, except 

 for establishing that the property does not reside in the i region, 

 Horiuchi has not mapped it more accurately. 



Conclusion 



One obvious explanation of the ability of these strains to make so 

 much ^-galactosidase is that there has been some kind of a duplication 

 of the genetic determinant for this enzyme. One might expect that if 

 the proportionate number of copies of a given gene is increased over 

 the numbers of the other genes, relatively more of the enzyme de- 

 termined by this gene will be produced. This appears to be the case 

 for bacteria carrying extra copies of the lac genetic region attached to 

 the fertility factor F, where a three- or four-fold increase of the /3-galac- 

 tosidase level has been reported (Jacob, Perrin, Sanchez, and Monod, 

 1960). 



The increase in the number of gene copies might occur either by an 



