Experimental evolution of propanediol oxidoreductase in Escherichia coli comparative analysis of the wild-type and mutant enzymes

A model for the study of experimental evolution is provided by the novel metabolic system responsible for the progressive utilization of l-1,2-propanediol by mutants of Escherichia coli (strains 3 and 430). In these mutant strains, propanediol oxidoreductase, which serves as l-lactaldehyde reductase in fucose fermentation by wild-type cells, became a key enzyme for aerobic catabolism of propanediol. In the wild-type strain (strain 1), the enzyme is inducible only anaerobically; in strains 3 and 430, the enzyme is synthesized constitutively even in the presence of air. The propanediol oxidoreductase from all three strains was purified to homogeneity by the same procedure. The enzyme of strain 3 clearly differed from that of strain 1 in several respects: K_m and V in both directions of the reaction, energy of activation, thermal stability, pH optimum and substrate specificity. However, no difference in any of the above characteristics was found between the enzymes of strains 3 and 430. All three enzymes presented the same electrophoretic mobility. According to immunological data, all three strains differed in their intracellular enzyme level.