BACTERL\ WITH HIGH LEVELS OF SPECIFIC ENZYMES 99 



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GENERATIONS 



Figure 3. Rise in ^-galactosidase activity of a strain of bacteria (E102) growing in a 

 lactose-controlled chemostat. Enzyme activity is given in units of millimicromoles of 

 ONPG hydrolyzed per minute per microgram of bacterial nitrogen at 28° C. at pH 7 in 

 M/10 sodium phosphate buffer. 



a constitutive strain, is there eventual selection of a strain with an 

 altered enzyme having a higher affinity for lactose or a higher turnover 

 number? Or will selection favor strains which simply have larger quan- 

 tities of the enzymes needed for lactose consumption? 



Such an investigation was started, and by now a number of cases 

 have been observed m which bacteria have been grown for long periods 

 at growth-rate-limiting concentrations of lactose. The general pattern 

 of results is like that illustrated by the experiment described in Figure 

 3. In this case an E. coli K-12 strain, E102, was inoculated into a 

 lactose-limited chemostat and cultured there for over 80 generations. 

 (This strain is an Hfr variant of E. coli K-12 isolated by J. Tomizawa 

 from the CavalH Hfr, CS 101, described by Garen and Skaar [1958]). 

 E102 is genetically i+z+y+ St5Pr+T6^ i.e., it forms /3-galactosidase (z+) 

 and galactoside permease ( y+ ) only when a suitable inducer is present 

 (i+); it is sensitive to streptomycin (St^), can make its own prohne 

 (Pr+), and is sensitive to phage T6 (TG^). At first a relatively low 



