The coexistence of two pathways for the metabolism of 2-hydroxymuconic semialdehyde in a naphthalene-grown pseudomonad

Biodegradation of phenols by the alga Ochromonas danica

The eukaryotic alga Ochromonas danica, a nutritionally versatile, mixotrophic chrysophyte, grew on phenol as the sole carbon source in axenic culture and removed the phenol carbon from the growth medium. Respirometric studies confirmed that the enzymes involved in phenol catabolism were inducible and that the alga oxidized phenol; the amount of oxygen consumed per mole of oxidized substrate was approximately 65% of the theoretical value. [U-14C]phenol was completely mineralized, with 65% of the 14C label appearing as 14CO2, approximately 15% remaining in the aqueous medium, and the rest accounted for in the biomass. Analysis of the biomass showed that 14C label had been incorporated into the protein, nucleic acid, and lipid fractions; phenol carbon is thus unequivocally assimilated by the alga. Phenol-grown cultures of O. danica converted phenols to the corresponding catechols, which were further metabolized by the meta-cleavage pathway. This surprising result was rigorously confirmed by taking the working stock culture through a variety of procedures to check that it was axenic and repeating the experiments with algal extracts. This is, as far as is known, the first definitive identification of the meta-cleavage pathway for aromatic ring degradation in a eukaryotic alga, though its incidence in other eukaryotes has been (infrequently) suggested.

Oxidative Pathway for the Biodegradation of Nitrobenzene by Comamonas sp. Strain JS765

Previous studies have shown that the biodegradation of nitrobenzene by Pseudomonas pseudoalcaligenes JS45 proceeds by the reduction of nitrobenzene through nitrosobenzene and hydroxylaminobenzene, followed by rearrangement to 2-aminophenol, which then undergoes meta ring cleavage. We report here the isolation of a Comamonas sp. that uses an oxidative pathway for the complete mineralization of nitrobenzene. The isolate, designated strain JS765, uses nitrobenzene as a sole source of carbon, nitrogen, and energy. Nitrobenzene-grown cells oxidized nitrobenzene, with the stoichiometric release of nitrite. Extracts of nitrobenzene-grown JS765 showed high levels of catechol 2,3-dioxygenase activity that were not abolished by heating the cell extracts to 60(deg)C for 10 min. The ring cleavage product had an absorbance maximum at 375 nm, consistent with that of 2-hydroxymuconic semialdehyde. Both NAD-dependent dehydrogenase and NAD-independent hydrolase activities towards 2-hydroxymuconic semialdehyde were induced in extracts of nitrobenzene-grown cells. Catechol accumulated in the reaction mixture when cells preincubated with 3-chlorocatechol were incubated with nitrobenzene. Conversion of nitrobenzene to catechol by induced cells in the presence of 3-chlorocatechol and (sup18)O(inf2) demonstrated the simultaneous incorporation of two atoms of oxygen, which indicated that the initial reaction was dioxygenation. The results indicate that the catabolic pathway involves an initial dioxygenase attack on nitrobenzene with the release of nitrite and formation of catechol, which is subsequently degraded by a meta cleavage pathway.

Evolutionary relationships between catabolic pathways for aromatics: conservation of gene order and nucleotide sequences of catechol oxidation genes of pWW0 and NAH7 plasmids

TOL plasmid pWW0 and plasmid NAH7 encode catabolic enzymes required for oxidative degradation of toluene and naphthalene, respectively. The gene order of the catabolic operon of NAH7 for salicylate oxidation was determined to be: promoter--nahG (the structural gene for salicylate hydroxylase)--nahH (catechol 2.3-dioxygenase)--nahI (hydroxymuconic semialdehyde dehydrogenase)--nahN (hydroxymuconic semialdehyde hydrolase)--nahL (2-oxopent-4-enoate hydratase). This order is identical to that of the isofunctional genes of TOL plasmid pWW0. The complete nucleotide sequence of nahH was determined and compared with that of xylE, the isofunctional gene of TOL plasmid pWW0. There were 20% and 16% differences in their nucleotide and amino acid sequences, respectively. The homology between the NAH7 and TOL pWW0 plasmids ends upstream of the Shine-Dalgarno sequences of nahH and xylE, but the homology continues downstream of these genes. This observation suggested that genes for the catechol oxidative enzymes of NAH7 and TOL pWW0 were derived from a common ancestral sequence which was transferred as a discrete segment of DNA between plasmids.

Roles of the divergent branches of the meta-cleavage pathway in the degradation of benzoate and substituted benzoates

The TOL plasmid-specified meta-cleavage pathway for the oxidative catabolism of benzoate and toluates branches at the ring cleavage products of catechols and reconverges later at 2-oxopent-4-enoate or its corresponding substituted derivatives. The hydrolytic branch of the pathway involves the direct formation of 2-oxopent-4-enoate or its derivatives, whereas the oxalocrotonate branch involves three enzymatic steps effected by a dehydrogenase, an isomerase, and a decarboxylase, which produce the same compounds. Evidence is presented which shows that benzoate and p-toluate can, under certain circumstances, be catabolized by the hydrolytic branch. However, in a fully functional pathway, only m-toluate is dissimilated via this branch, and benzoate and p-toluate are catabolized almost exclusively by the oxalocrotonate branch. The biochemical basis of this selectivity was found to reside in the high affinity of the dehydrogenase for ring fission products derived from benzoate and p-toluate and its inability to attack the ring fission product derived from m-toluate. Although isomerization of 4-oxalocrotonate occurs spontaneously in vitro, enzymatic isomerization was found to be essential for effective functioning of this branch of the pathway in vivo.

Evidence for isofunctional enzymes in the degradation of phenol, m- and p-toluate, and p-cresol via catechol meta-cleavage pathways in Alcaligenes eutrophus

A study of the degradation of phenol, p-cresol, and m- and p-toluate by Alcaligenes eutrophus 345 has provided evidence that these compounds are metabolized via separate catechol meta-cleavage pathways. Analysis of the enzymes synthesized by wild-type and mutant strains and by strains cured of the plasmid pRA1000, which encodes m- and p-toluate degradation, indicated that two or more isofunctional enzymes mediated several steps in the pathway. The formation of three catechol 2,3-oxygenases and two 2-hydroxymuconic semialdehyde hydrolases was indicated from an examination of the ratio of the specific activities of these enzymes against various substrates. Evidence for two 2-hydroxymuconic semialdehyde dehydrogenases, two 4-oxalocrotonate isomerases and decarboxylases, and three 2-ketopent-4-enoate hydratases was derived from the induction of these enzymes under different growth conditions. Each activity was detected when the wild type was grown in the presence of m-toluate, but not when grown with phenol (except for a hydratase) or p-cresol, whereas in strains cured of pRA1000, growth with phenol or p-cresol, but not with m-toluate, induced these enzymes. Hydroxylation of phenol and p-cresol appears to be mediated by the same enzyme.

Metabolism of allylglycine and cis-crotylglycine by Pseudomonas putida (arvilla) mt-2 harboring a TOL plasmid

Spontaneous mutants which acquired the ability to utilize d-allylglycine (d-2-amino-4-pentenoic acid) and dl-cis-crotylglycine (dl-2-amino-cis-4-hexenoic acid) but not l-allylglycine or dl-trans-crotylglycine could be readily isolated from Pseudomonas putida mt-2 (PaM1). Derivative strains of PaM1 putatively cured of the TOL (pWWO) plasmid were incapable of forming mutants able to utilize the amino acids for growth; however, this ability could be regained by conjugative transfer of the TOL (pWWO) plasmid from a wild-type strain of mt-2 or of the TOL (pDK1) plasmid from a related strain of P. putida (HS1), into cured recipients. dl-Allylglycine-grown cells of one spontaneous mutant (PaM1000) extensively oxidized dl-allylglycine and dl-cis-crotylglycine, whereas only a limited oxidation was observed toward l-allylglycine and dl-trans-crotylglycine. Cell extracts prepared from PaM1000 cells contained high levels of 2-keto-4-hydroxyvalerate aldolase and 2-keto-4-pentenoic acid hydratase, the latter enzyme showing higher activity toward 2-keto-cis-4-hexenoic acid than toward the trans isomer. Levels of other enzymes of the TOL degradative pathway, including toluate oxidase, catechol-2,3-oxygenase, 2-hydroxymuconic semialdehyde hydrolase, and 2-hydroxymuconic semialdehyde dehydrogenase, were also found to be elevated after growth on allylglycine. Whole cells of a putative cured strain, PaM3, accumulated 2-keto-4-pentenoic acid from d-allylglycine, which was shown to be rapidly degraded by cell extracts of PaM1000 grown on dl-allylglycine. These same cell extracts were also capable of catalyzing the dehydrogenation of d- but not l-allylglycine and were further found to metabolize the amino acid completely to pyruvate and acetaldehyde. Differential centrifugation of crude cell extracts localized d-allylglycine dehydrogenase activity to membrane fractions. The results are consistent with a catabolic pathway for d-allylglycine and dl-cis-crotylglycine involving the corresponding keto-enoic acids as intermediates, the further metabolism of which is effected by the action of TOL plasmid-encoded enzymes.

Role and regulation of the ortho and meta pathways of catechol metabolism in pseudomonads metabolizing naphthalene and salicylate

The enzymes of naphthalene metabolism are induced in Pseudomonas putida ATCC 17484, PpG7, NCIB 9816, and PG and in Pseudomonas sp. ATCC 17483 during growth on naphthalene or salicylate; 2-aminobenzoate is a gratuitous inducer of these enzymes. The meta-pathway enzymes of catechol metabolism are induced in ATCC 17483 and PPG7 during growth on naphthalene or salicylate or during growth in the presence of 2-aminobenzoate, but in ATCC 17484 and NCIB 9816 the ortho-pathway enzymes of catechol metabolism are induced during growth on naphthalene or salicylate. 2-Aminobenzoate does not induce any enzymes of catechol metabolism in the latter two organisms. In Pseudomonas PG the meta-pathway enzymes are present at high levels under all conditions of growth, but this organism and PpG7 can induce ortho-pathway enzymes during naphthalene or salicylate metabolism. Salicylate appears to be the inducer of the enzymes of naphthalene metabolism in all of the organisms studied and, where they are inducible, of the meta-pathway enzymes, but the properties of Pseudomonas PG suggest that separate, regulatory systems may exist.

Metabolism of naphthalene, 2-methylnaphthalene, salicylate, and benzoate by Pseudomonas PG: regulation of tangential pathways

Naphthalene is metabolized by Pseudomonas PG through 1,2-dihydroxynaphthalene and salicylate to catechol, which is then degraded by the meta pathway. 2-Methylnaphthalene, but not 1-methylnaphthalene, also serves as a growth substrate and is metabolized by the same route, through 4-methylcatechol. The same nonspecific meta pathway enzymes appear to be induced by growth on either naphthalene or 2-methylnaphthalene. The level to which 2-hydroxymuconic semialdehyde hydrolase is induced is low and probably of no metabolic significance. Growth on salicylate or catechol, both intermediates of naphthalene degradation, or benzoate results in induction of the ortho pathway, the alternative route for catechol dissimilation. No induction of 1,2-dihydroxynaphthalene oxygenase was found in salicylate-grown cells. Anaerobic growth on a succinate-nitrate medium in the presence of various inducers indicates that cis, cis-muconate, or one of its metabolites is the inducer of the ortho pathway enzymes. The inducer or inducers of the early enzymes of naphthalene degradation and of the meta pathway enzymes must be an early intermediate of the naphthalene pathway above salicylate.

Metabolism of phenol and cresols by mutants of Pseudomonas putida

Mutant strains of Pseudomonas putida strain U have been obtained which are deficient in enzymes of the degradative pathways of phenol and cresols. Mutant strains deficient in catechol 2, 3-oxygenase accumulated the appropriate catechol derivative from cresols. A mutant strain which would not grow on either phenol or a cresol was shown to be deficient in both 2-hydroxymuconic semialdehyde hydrolase and a nicotinamide adenine dinucleotide, oxidized form, (NAD(+))-dependent aldehyde dehydrogenase. When this strain was grown in the presence of phenol or a cresol, the appropriate product of meta fission of these compounds accumulated in the growth medium. A partial revertant of this mutant strain, which was able to grow on ortho- and meta-cresol but not para-cresol, was shown to have regained only the hydrolase activity. This strain was used to show that the products of meta ring fission of the cresols and phenol are metabolized as follows: (i) ortho- and meta-cresol exclusively by a hydrolase; (ii) para-cresol exclusively by a NAD(+)-dependent aldehyde dehydrogenase; (iii) phenol by both a NAD(+)-dependent dehydrogenase and a hydrolase in the approximate ratio of 5 to 1. This conclusion is supported by the substrate specificity and enzymatic activity of the hydrolase and NAD(+)-dependent aldehyde dehydrogenase enzymes of the wild-type strain. The results are discussed in terms of the physiological significance of the pathway. Properties of some of the mutant strains isolated are discussed.

Stereospecific enzymes in the degradation of aromatic compounds by pseudomonas putida

Two reactions in the catabolism of catechol by meta-fission, namely, hydration of 2-oxopent-4-enoate (vinylpyruvate) and aldol fission of the product, are catalyzed by stereospecific enzymes. The absolute configuration of this hydration product was shown to be l(S)-4-hydroxy-2-oxopentanoate. Vinylpyruvate hydratase, purified almost to homogeneity, had a molecular weight of about 287,000 and was dissociated in sodium dodecyl sulfate, without prior treatment with mercaptoethanol, into a species with an approximate molecular weight of 28,000. The hydratase was highly specific for its substrates; thus, although 2-oxo-cis-hex-4-enoate was also hydrated, structurally similar compounds such as the trans isomer, vinylacetic and crotonic acids, and the ring-fission products of catechol and methylcatechols were not attacked. Vinylpyruvate hydratase was activated by Mn(2+) ions. On the basis of these observations, a mechanism is proposed which closely resembles that for 4-hydroxy-2-oxopentanoate aldolase. A possible evolutionary connection between functionally related, divalent cation-activated hydro-lyases and aldolases is discussed. It was also demonstrated that l-(S)-4-hydroxy-2-oxohexanoate is the biologically active enantiomer of this hydroxy acid.

Degradation of protocatechuate in Pseudomonas testosteroni by a pathway involving oxidation of the product of meta-fission

In addition to catalyzing the hydrolysis of 4-carboxy-2-hydroxymuconic semialdehyde, formed by meta-fission of protocatechuate, Pseudomonas testosteroni also possesses a nicotinamide adenine dinucleotide(phosphate)-linked dehydrogenase for this compound and can degrade protocatechuate to pyruvate and oxaloacetate.