We isolated an Escherichia coli mutant strain that suppresses the glycolate-negative phenotype of a strain deficient in both GlcA and LldP transporters of this compound. This suppressing phenotype was assigned to yjcG, a gene whose function was previously unknown, which was found to encode a membrane protein able to transport glycolate. On the basis of sequence similarity, the yjcG gene product was classified as a member of the sodium:solute symporter family. Northern experiments revealed that yjcG is cotranscribed with its neighbor, acs, encoding acetyl coenzyme A synthetase, which is involved in the scavenging acetate. The fortuitous presence of an IS2 element in acs, which impaired yjcG expression by polarity in our parental strain, allowed us to conclude that the alternative glycolate carrier became active after precise excision of IS2 in the suppressed strain. The finding that yjcG encodes a putative membrane carrier for glycolate and the cotranscription of yjcG with acs suggested that the primary function of the yjcG gene product (proposed gene name, actP) could be acetate transport and allowed us to define an operon involved in acetate metabolism. The time course of [1,2-14 C]acetate uptake and the results of a concentration kinetics analysis performed with cells expressing ActP or cells deficient in ActP supported the the hypothesis that this carrier is an acetate transporter and suggested that there may be another transport system for this monocarboxylate.

The FucO protein, a member of the group III "iron-activated" dehydrogenases, catalyzes the interconversion between L-lactaldehyde and L-1,2-propanediol in Escherichia coli. The three-dimensional structure of FucO in a complex with NAD ؉ was solved, and the presence of iron in the crystals was confirmed by X-ray fluorescence. The FucO structure presented here is the first structure for a member of the group III bacterial dehydrogenases shown experimentally to contain iron. FucO forms a dimer, in which each monomer folds into an ␣/␤ dinucleotide-binding N-terminal domain and an all-␣-helix C-terminal domain that are separated by a deep cleft. The dimer is formed by the swapping (between monomers) of the first chain of the ␤-sheet. The binding site for Fe 2؉ is located at the face of the cleft formed by the C-terminal domain, where the metal ion is tetrahedrally coordinated by three histidine residues (His200, His263, and His277) and an aspartate residue (Asp196). The glycine-rich turn formed by residues 96 to 98 and the following ␣-helix is part of the NAD ؉ recognition locus common in dehydrogenases. Site-directed mutagenesis and enzyme kinetic assays were performed to assess the role of different residues in metal, cofactor, and substrate binding. In contrast to previous assumptions, the essential His267 residue does not interact with the metal ion. Asp39 appears to be the key residue for discriminating against NADP ؉ . Modeling L-1,2-propanediol in the active center resulted in a close approach of the C-1 hydroxyl of the substrate to C-4 of the nicotinamide ring, implying that there is a typical metal-dependent dehydrogenation catalytic mechanism.In Escherichia coli and other enterobacteria the anaerobic metabolism of L-fucose and L-rhamnose requires the enzyme lactaldehyde:propanediol oxidoreductase (FucO), which is encoded by the fucO gene of the fucose regulon (6,14,16,24,32). The breakdown of these methylpentoses generates the intermediate metabolite L-lactaldehyde, which under anaerobic conditions, with NADH as a cofactor, is reduced by FucO to L-1,2-propanediol, which is excreted as a fermentation product (14). In mutant strains of E. coli adapted to grow on L-1,2-propanediol, FucO catalyzes the oxidation of the polyol to L-lactaldehyde, which is subsequently oxidized to L-lactate by a specific aldehyde dehydrogenase (41) and introduced into the general metabolism. FucO, which is induced regardless of the respiratory conditions of the culture, remains fully active in the absence of oxygen (11). In the presence of oxygen, this enzyme becomes oxidatively inactivated by a metal-catalyzed oxidation mechanism (10).FucO is an iron-dependent metalloenzyme that is inactivated by other metals, such as zinc, copper, or cadmium (40), and has been reported to be a homodimer formed by monomers consisting of 383 amino acids and having a molecular mass of 40,644 Da. The iron in the active center accounts for the oxidative inactivation of FucO mentioned above (10). A putative iron-binding motif encompassing a 15-amino-ac...

The locus glc (min 64.5), associated with the glycolate utilization trait in Escherichia coli, is known to contain glcB, encoding malate synthase G, and the gene(s) needed for glycolate oxidase activity. Subcloning, sequencing, insertion mutagenesis, and expression studies showed five additional genes: glcC and in the other direction glcD, glcE, glcF, and glcG followed by glcB. The gene glcC may encode the glc regulator protein. Consistently a chloramphenicol acetyltransferase insertion mutation abolished both glycolate oxidase and malate synthase G activities. The proteins encoded from glcD and glcE displayed similarity to several flavoenzymes, the one from glcF was found to be similar to iron-sulfur proteins, and that from glcG had no significant similarity to any group of proteins. The insertional mutation by a chloramphenicol acetyltransferase cassette in either glcD, glcE, or glcF abolished glycolate oxidase activity, indicating that presumably these proteins are subunits of this enzyme. No effect on glycolate metabolism was detected by insertional mutation in glcG. Northern (RNA) blot experiments showed constitutive expression of glcC but induced expression for the structural genes and provided no evidence for a single polycistronic transcript.

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When grown anaerobically on L-rhamnose, Salmonella typhimurium excreted 1,2-propanediol as a fermentation product. Upon exhaustion of the methyl pentose, 1,2-propanediol was recaptured and further metabolized, provided the culture was kept under anaerobic conditions. n-Propanol and propionate were found in the medium as end products of this process at concentrations one-half that of 1,2-propanediol. As in Klebsiella pneumoniae (T. Toraya, S. Honda, and S. Fukui, J. Bacteriol. 139:3947, 1979), a diol dehydratase which transforms 1,2-propanediol to propionaldehyde and the enzymes involved in a dismutation that converts propionaldehyde to n-propanol and propionate were induced in S. typhimurium cultures able to transform 1,2-propanediol anaerobically.In Escherichia coli, L-rhamnose is metabolized by the sequential action of a rhamnose permease that transports the sugar across the membrane, a rhamnose isomerase (20) that converts it to L-rhamnulose, a rhamnulose kinase (21) that phosphorylates the L-rhamnulose to L-rhamnulose 1-phosphate, and an aldolase (5) that cleaves the L-rhamnulose 1-phosphate into dihydroxyacetone phosphate and L-lactaldehyde. Although no detailed description of the enzymes has been reported, the same pathway has been described in Salmonella typhosa (7) and Salmonella typhimurium (1).Under anaerobic conditions, E. coli induces an NADHdependent oxidoreductase that reduces L-lactaldehyde to 1,2-propanediol, which is excreted into the medium (4). On the basis of propanediol oxidoreductase activity induction and propanediol excretion, this fermentation mechanism has also been proposed for S. typhimurium (3). However, 1,2-propanediol, which is not further metabolized in anaerobic E. coli cultures, gradually disappears from the medium in S. typhimurium cultures maintained under similar anaerobic conditions (J. Badia, unpublished observation).Aerobic metabolism of 1,2-propanediol has been described for several species (9, 13) and for mutant cells of E. coli (18) able to grow on the diol. In all cases the first step is the oxidation to lactaldehyde through the action of an oxidoreductase. Lactaldehyde is subsequently metabolized to lactate and pyruvate (6,17). In contrast, anaerobic metabolism of 1,2-propanediol in Klebsiella pneumoniae (22) or Clostridium glycolicum (8) has been reported to be mediated by a coenzyme B12-dependent diol dehydratase that yields propionaldehyde, which is immediately metabolized by a dismutation to n-propanol and propionate.In this report we characterize the 1,2-propanediol transformation that occurs in S. typhimurium cultures and establish the metabolic pathway involved.