An evolvant of Escherichia coli that employs the L-fucose pathway also for growth on L-galactose and D-arabinose.

L-Galactose, D-arabinose, and L-fucose form six-membered rings with identical stereoconfigurations. However, only L-fucose can serve as the sole carbon and energy source of wild-type Escherichia coli K-12. A mutant that can grow on L-galactose and D-arabinose was isolated by alternate selection on the two sugars. The L-fucose pathway became inducible by all three sugars. Transduction into the mutant of the wild-type fuc+ region containing both the regulatory and structural genes abolished the novel growth abilities on L-galactose and D-arabinose, whereas transduction into the mutant of a fuc deletion abolished the growth abilities on all three sugars. Introduction of the wild-type fucR+ (which encodes the activator protein for the fuc regulon) on a multicopy plasmid depressed the growth abilities of the mutant on L-galactose and D-arabinose, but not on L-fucose. The results suggest that the effector specificity of the activator protein in the mutant was broadened. It is proposed that an adaptive response of an activator-controlled system is more likely than that of a repressor-controlled system to achieve fixation in a population, because the first variant to emerge in response to a novel metabolic demand has a good chance of having an altered specificity of regulation. Such a change entails little or no metabolic liability during the absence of the novel substrate. In contrast, the first variant of a negatively controlled system to emerge has an overwhelming chance of being the result of a random mutation that destroys repressor function.(ABSTRACT TRUNCATED AT 250 WORDS)