Increase in the catalytic rate of beta-galactosidase by selection in chemostats at changing dilution rates

The experiments presented in this paper explore whether mutants with catalytically more active beta-galactosidase (E.C. 3.2.1.23) can be selected in lactose-limited chemostats. This experimental system has been chosen because lactose metabolism in Escherichia coli is well understood both from a biochemical and genetic point of view. In a lactose-limited chemostat with constant dilution rates, both beta-galactosidase and the lactose permease increased in quantity. The catalytic rate (kcat) of beta-galactosidase in populations showed no change. On the other hand, in chemostats with periodically changing dilution rates, the catalytic rate of beta-galactosidase increased dramatically. Therefore, the selection for beta-galactosidase with improved catalytic rate occurs in chemostats with fluctuating dilution rates but not in ones with constant dilution. These observations may be of value in the selection of other enzymes with enhanced catalytic rates.