Disruption of the fucose pathway as a consequence of genetic adaptation to propanediol as a carbon source in Escherichia coli

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Abstract
In E. coli, L-fucose is dissimilated via an inducible pathway mediated by L-fucose permease, L-fucose isomerase, L-fuculose kinase and L-fuculose 1-phosphate aldolase. The last enzyme cleaves the 6-carbon substrate into dihydroxyacetone phosphate and L-lactaldehyde. Aerobically, lactaldehyde is oxidized to L-lactate by a NAD-linked dehydrogenase. Anaerobically, lactaldehyde is reduced by an NADH-coupled reductase to L-1,2-propanediol, which is lost into the medium irretrievably, even when oxygen is subsequently introduced. Propanediol excretion is thus the end result of a dismutation that permits further anaerobic metabolism of dihydroxyacetone phosphate. A mutant selected for its ability to grow aerobically on propanediol as a carbon and energy source was reported to produce lactaldehyde reductase constitutively and at high levels, even aerobically. Under the new situation, this enzyme serves as a propanediol dehydrogenase. It was also reported that the mutant had lost the ability to grow on fucose. In the present study, it is shown that in wild-type cells the full synthesis of lactaldehyde dehydrogenase requires the presence of both molecular oxygen and a small molecule effector, and the full synthesis of lactaldehyde reductase requires anaerobiosis and the presence of a small molecule effector. The failure of mutant cells to grow on fucose reflects the impairment of a regulatory element in the fucose system that prevents the induction of the permease, the isomerase and the kinase. The aldolase, on the other hand, is constitutively synthesized. Three independent fucose-utilizing revertants of the mutant all produce the permease, the isomerase, the kinase, as well as the aldolase, constitutively. These strains do not grow as well as the parental mutant on propanediol.