Regulation of fatty acid degradation in Escherichia coli: analysis by operon fusion

Isolation and genetic analysis of mutations allowing the degradation of furans and thiophenes by Escherichia coli

Successive mutations of Escherichia coli yielded a strain that was able to degrade a variety of heterocyclic oxygen- and sulfur-containing ring compounds. In particular, this strain could use both furan-2-carboxylic acid and thiophene-2-carboxylic acid as sole carbon and energy sources. Nitrogen-containing heterocyclic compounds were not degraded. This mutant was isolated by selecting first for oxidation of furan derivatives and then for thiophene degradation. Genetic analysis revealed that mutations in three novel genes, thdA (12 min), thdC (92 min), and thdD (98 min), were required for thiophene degradation. In addition, constitutively at both of the previously characterized fadR and atoC loci was required for efficient thiophene breakdown. The pathway of furan and thiophene degradation remains obscure, but the inability of our mutants to degrade 5-nitro- or 5-bromo-substituted furan derivatives suggests that hydroxylation at position 5 may be involved. Thiophene derivatives were toxic when they were present at concentrations of 0.1% or greater; however, addition of trace amounts of phenylalanine plus tyrosine greatly reduced this effect.

A molecular view of fatty acid catabolism in Escherichia coli

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Acetohydroxy acid synthase I, a required enzyme for isoleucine and valine biosynthesis in Escherichia coli K-12 during growth on acetate as the sole carbon source

Escherichia coli K-12 has two acetohydroxy acid synthase (AHAS) isozymes (AHAS I and AHAS III). Both of these isozymes catalyze the synthesis of alpha-aceto-alpha-hydroxybutyrate and alpha-acetolactate, which are key intermediates of the isoleucine-valine biosynthetic pathway. Strains lacking either isozyme but not both activities have been previously shown to grow well in minimal media in the absence of isoleucine and valine on any of several commonly used carbon sources (e.g., glucose or succinate). We report the characterization of mutants that were unable to grow on either acetate or oleate as a sole carbon source due to a defect in isoleucine-valine biosynthesis. The defect in isoleucine-valine biosynthesis was expressed only on these carbon sources and was due to the loss of AHAS I activity, resulting from lesions in the ilvBN operon. Previously identified ilvBN mutant strains also failed to grow on acetate or oleate minimal media. Our results indicated that AHAS I is an essential enzyme for isoleucine and valine biosynthesis when E. coli K-12 is grown on acetate or oleate as the sole carbon source. AHAS III was expressed during growth on acetate or oleate but was somehow unable to produce sufficient amounts of alpha-aceto-alpha-hydroxybutyrate and alpha-acetolactate to allow growth.

Cloning and characterization of a gene (fadR) involved in regulation of fatty acid metabolism in Escherichia coli

The regulatory gene fadR has been previously characterized by classical genetic means as a diffusible protein which exerts negative control over fatty acid degradation and acetate metabolism. fadR has also been implicated in the regulation of unsaturated fatty acid biosynthesis. To facilitate the identification of the product of the fadR gene and to study the mechanism by which this multifunctional regulatory gene exerts its control, we cloned a segment of DNA containing the fadR gene in the phage vector lambda L47. Subsequent subcloning of a segment of the chromosomal DNA from the lambda fadR+ phage into various plasmid vectors resulted in the isolation of the fadR gene on a 1.3-kilobase-pair HindIII-EcoRV fragment. fadR strains harboring the cloned fadR+ gene showed inducible levels of fatty acid oxidation and crotonase (enoyl-coenzyme A-hydratase, fadB) activity. The cloned gene exerted transcriptional control over beta-galactosidase synthesis in an fadR strain that had a lambda phi (fad-lacZ+) operon fusion. An fadR mutation in fabA(Ts) strains prevents growth at permissive temperatures without unsaturated fatty acid supplementation (Nunn et al., J. Bacteriol. 154:554-560, 1983). Plasmids carrying the fadR+ gene suppress this unsaturated fatty acid auxotrophy in fadR fabA(Ts) strains at the permissive condition. Maxicell analysis identified a 29,000-dalton protein encoded by the 1.3-kilobase fragment which appeared to be associated with functional fadR gene activity.

Benzyl derivative facilitation of transcription in Escherichia coli at the ara and lac operon promoters: metabolite gene regulation (MGR)

A number of benzyl derivatives have been tested for their ability to induce the expression of the araBAD operon in an Escherichia coli K-12 strain. Those derivatives shown to be stimulatory include: benzoic acid (BA), para-amino benzoic acid (PABA), para-hydroxy benzoic acid (PHBA), ortho-amino benzoic acid (OABA), 3-hydroxy-4-methoxy phenylethylamine (MTA), and 4-hydroxy-3-methoxyphenol acetic acid (HVA). The araC gene product was necessary to facilitate the induction. To further characterize if the inductive effect was mediated at the level of transcription, an araBAD-tetracycline resistant (Tcr) operon fusion plasmid (pAP-B) was employed. Benzyl derivatives which induce expression of the araBAD operon in situ also induced a Tcr phenotype with pAP-B. Both indole acetic acid (IAA) and imidazole (IM), which were previously shown to circumvent the necessity for cAMP in the induction of the araBAD operon, also induced a Tcr phenotype with pAP-B. Induction of lac or other cAMP responding operons with the inducing molecules at the chromosomal level was not detectable when assessed by carbon utilization. However, a lacZYA-Tcr operon fusion plasmid (pLPI) did respond to IAA and several of the inducing benzyl derivatives. Catabolite repression of chromosomal araBAD expression was reversed when the exogenous concentration of OABA was elevated. Similar effects on the Tcr phenotypes conferred by pAP-B and pLP1 were observed when OABA or several other inducing benzyl derivatives were present exogenously.

Regulation of fatty acid transport in Escherichia coli: analysis by operon fusion

The regulation of the fadL gene, which encodes a long-chain fatty acid (LCFA) transport component, was examined by constructing a strain of Escherichia coli K-12 that bears a phi (fadL-lacZ+) operon fusion plus a wild-type fadL gene. This merodiploid strain expressed LCFA transport and beta-galactosidase activity coordinately under noninducing, inducing, and catabolite-repressing conditions. Merodiploid strains which carried a defective fadR gene expressed LCFA transport and beta-galactosidase activity constitutively. These results suggest that expression of the fadL gene is regulated by the fadR gene and is inducible at the level of transcription by LCFA.