BACTERIA WITH HIGH LEVELS OF SPECIFIC ENZYMES 103 



preventing the growth of the strains with high enzyme levels, although 

 they are generally much less effective than lactose in this regard. 



Genetic basis of high production rates 



Genetic analysis of the strains with high levels of /3-galactosidase 

 is made difficult by their sensitivity to lactose: lactose-containing 

 media cannot be used to select lactose-positive bacteria resulting from 

 some genetic exchange, since most of the bacteria of high /?-galacto- 

 sidase would be lost. Some information about the genetic basis of the 

 high rate of ^-galactosidase production has nevertheless been obtained 

 by Tomizawa, using crosses betw een Hfr strains with high levels of the 

 enzyme and F~ strains. 



Genetic control of the formation of /?-galactosidase is determined 

 by a set of closely linked loci lying between a locus for proline require- 

 ment and one for resistance to bacteriophage T6. The most recent in- 

 terpretation of these loci and their functions has been given by Jacob, 

 Perrin, Sanchez, and Monod ( 1960 ) . They note that there is one gene 

 ( cistron ) , z, \\'hich provides the necessary information for the structure 

 of /^-galactosidase, and one gene, y, which provides for the structure of 

 the galactoside permease. Also, there is a gene, i (regulator), which by 

 its control of the formation of a specific repressor substance determines 

 whether the bacterium is inducible or constitutive for the synthesis of 

 yS-galactosidase and galactoside permease. To account for some of their 

 recent observations, these authors propose a fourth gene, a (operator), 

 as part of the control mechanism. 



In this scheme they imagine that the formation of /3-galactosidase 

 and galactoside permease is controlled in the following way. Genes ::: 

 and y are controlled as a unit ( operon ) in such a way that synthesis of 

 both /?-galactosidase and galactoside permease is turned off or on to- 

 gether. The on-off switching is controlled by the o gene (operator), 

 which determines the function only of the genes located in the operon 

 adjacent to it. The operator has no effect on the lac operon located on 

 another chromosome within the cell. The state of the o gene ( off or on ) 

 is controlled by the repressor substance, whose formation is determined 

 by the / gene. 



Let us use a symbol, H, to denote the attribute of making /?-galac- 

 tosidase: bacteria making normal levels of it will be labeled H+ and 

 those making high levels H~. 



In one case Tomizawa crossed an Hfr strain which was able to 

 make proline, was sensitive to phage T6 and to streptomycin, and was 

 constitutive for a high level of /3-galactosidase and presumably per- 

 mease, with an F- bacterium which required proline, was resistant to 



