L-1,2-Propanediol is an irretrievable end product of L-fucose fermentation by Escherichia coli. Selection for increased aerobic growth rate on propanediol results in the escalation of basal synthesis of the NAD+-linked oxidoreductase encoded by fucO, a member of thefuc regulon for the utilization of L-fucose. In general, when fucO becomes constitutively expressed, two other simultaneous changes occur: the fucA gene encoding fuculose-1-phosphate aldolase becomes constitutively expressed and the fucPIK operon encoding fucose permease, fucose isomerase, and fuculose kinase becomes noninducible. In the present study, we show that fucO and fucA form an operon which is divergently transcribed from the adjacent fucPIK operon. In propanediol-positive and fucose-negative mutants the cis-controliing region shared by the operonsfucAO and fucPIK is lengthened by 1.2 kilobases. DNA hybridization identified the insertion element to be IS5. This element, always oriented in the same direction with the left end (the BglII end) proximal to fucA, apparently causes constitutive expression offucAO and noninducibility of fucPIK. The DNA of the fucAO operon and a part of the adjacent fucP was sequenced.Following the discovery that a genetic derepression of ribitol dehydrogenase in Klebsiella pneumoniae conferred a growth ability on xylitol (27, 33), we tried to find other models of experimental evolution for assessing the importance of regulatory mutations in the acquisition of novel functions (for a review, see reference 30). During this search, we discovered that Escherichia coli can give rise to mutants that utilize L-1,2-propanediol aerobically as a sole source of carbon and energy. Serial selection on the compound resulted in the emergence of a mutant (ECL3) which grew on the novel carbon and energy source at a rate close to that on glycerol. This mutant produced constitutively an NAD+-linked oxidoreductase active on L-1,2-propanediol (46). The proximity of the locus specifying this enzyme activity and the fuc locus (at min 60) specifying the growth ability on L-fucose led us to ask whether there was a link between the metabolism of the two compounds. When the propanediol-positive mutant was tested with fucose as a carbon and energy source, growth no longer occurred (13).Previous work, together with our further studies (13), revealed the biochemical connection between the two metabolic pathways (Fig. 1). The dissimilation of fucose by wild-type E. coli requires the sequential action of fucose permease (encoded byfucP), fucose isomerase (encoded by fucf), fucose kinase (encoded by fucK), and fuculose-lphosphate aldolase (encoded by fucA). The last enzyme catalyzes the formation of dihydroxyacetone phosphate and L-lactaldehyde. Under anaerobic conditions, the aldehyde is reduced to L-1,2-propanediol by an NAD+-linked oxidoreductase (encoded by fucO) and excreted into the medium. Under aerobic conditions, the aldehyde is oxidized to Llactate and then to pyruvate which enters the general metabolic pool. The inducer of the fucose syst...

Dissimilation of L-fucose as a carbon and energy source by Escherichia coli involves a permease, an isomerase, a kinase, and an aldolase encoded by the fuc regulon at minute 60.2. Utilization of L-rhamnose involves a similar set of proteins encoded by the rha operon at minute 87.7. Both pathways lead to the formation of L-lactaldehyde and dihydroxyacetone phosphate. A common NAD-linked oxidoreductase encoded by fucO serves to reduce L-lactaldehyde to L-1,2-propanediol under anaerobic growth conditions, irrespective of whether the aldehyde is derived from fucose or rhamnose. In this study it was shown that anaerobic growth on rhamnose induces expression of not only the fucO gene but also the entirefuc regulon. Rhamnose is unable to induce the fuc genes in mutants defective in rhaA (encoding L-rhamnose isomerase), rhaB (encoding L-rhamnulose kinase), rhaD (encoding L-rhamnulose 1-phosphate aldolase), rhaR (encoding the positive regulator for the rha structural genes), or fucR (encoding the positive regulator for the fuc regulon). Thus, cross-induction of the L-fucose enzymes by rhamnose requires formation of L-lactaldehyde; either the aldehyde itself or the L-fuculose 1-phosphate (known to be an effector) formed from it then interacts with the fucR-encoded protein to induce the fuc regulon.L-Fucose and L-rhamnose are dissimilated by Escherichia coli in parallel ways (Fig. 1). The trunk pathway for each compound is mediated by a permease (21; J. Power, personal communication), an isomerase (18, 41, 44), a kinase (12,23,42,45), and an aldolase (13,14,17,35,36). The two sugars differ in the stereoconfiguration at carbons 2 and 4, but structural differences in the intermediates disappear with cleavage of the phosphorylated ketose by the aldolase, yielding, in both cases, dihydroxyacetone phosphate and L-lactaldehyde.Aerobically, L-lactaldehyde is converted by an NADlinked dehydrogenase to L-lactate, which is oxidized to pyruvate for further metabolism (15, 39); anaerobically, L-lactaldehyde is reduced to L-1,2-propanediol, which is excreted into the medium (15, 40). The sacrifice of the aldehyde as a hydrogen sink increases the portion of dihydroxyacetone phosphate that can be utilized as a carbon and energy source.The catalytic proteins in each trunk pathway and the corresponding positive regulatory protein are encoded by a single gene cluster: thefuc locus at minute 60.2 (1,7,16,37,38) and the rha locus at minute 87.7 (1, 34). The structural genes of the fuc system appear to be organized as a regulon comprising at least three operons (7,(20)(21)(22), with L-fuculose 1-phosphate as the effector (3). The rha system responds to L-rhamnose as the effector (34).A common enzyme of broad function, encoded by the ald gene, which is linked to neither the fuc nor the rha locus, is responsible for the dehydrogenation of L-lactaldehyde to