METABOLIC FLUX AND FITNESS

  • 1 January 1987
    • journal article
    • research article
    • Vol. 115  (1) , 25-31
Abstract
Studies of Escherichia coli under competition for lactose in chemostat cultures have been used to determine the selective effects of variation in the level of the .beta.-galactoside permease and the .beta.-galactosidase enzyme. Theresults determine the adaptive topography of these gene products relative to growth in limiting lactose and enable predictions concerning the selective effects of genetic variants found in natural populations. In the terms of metabolic control theory, the .beta.-galactosidase enzyme at wild-type-induced levels has a small control coeffient with respect to fitness (C = 0.018), and hence genetic variants resulting in minor changes in enzyme activity have disproportionately small effects of fitness. However, the appeared control coefficient of the .beta.-galactoside permease at wild-type-induced levels is large (C = 0.551), and hence even minor changes in activity affect fitness. Therefore, we predict that genetic polymorphisms in the lacZ gene are subject to less effective selection in natural populations than are those in the lacY gene. The .beta.-galactoside permease is also less efficient than might be expected, and possible forces resulting in selection for an intermediate optimum level of permease activity are considered. The selective forces that maintain the lactose operon in a regulated state in natural populations also discussed.