Branching of o-nitrobenzoate degradation pathway in Arthrobacter protophormiae RKJ100: identification of new intermediates

We have earlier reported a novel reductive pathway for o-nitrobenzoate (ONB) degradation (at 0.5 mM) in Arthrobacter protophormiae RKJ100, which proceeds via the formation of o-hydroxylaminobenzoate (HABA) and anthranilate (AA). During growth of this organism at 40 times higher concentration (20 mM) of ONB, 3-hydroxyanthranilate (HAA) was identified as an intermediate by thin layer chromatography, gas chromatography and high performance liquid chromatography studies. Crude cell extracts of ONB-grown cells showed HAA 3,4-dioxygenase activity suggesting HAA as a terminal aromatic intermediate of the catabolic energy-yielding pathway as shown before in Pseudomonas fluorescens strain KU-7. HAA is further cleaved to 2-amino-3-carboxymuconic-6-semialdehyde by the action of HAA 3,4-dioxygenase. In this report we propose that ONB degradation occurs via the formation of HABA and the pathway branches at this point to form the two different aromatic intermediates AA and HAA by the action of a reductase and a mutase, respectively.     

Prokaryotic homologs of the eukaryotic 3-hydroxyanthranilate 3,4-dioxygenase and 2-amino-3-carboxymuconate-6-semialdehyde decarboxylase in the 2-nitrobenzoate degradation pathway of Pseudomonas fluorescens strain KU-7

The 2-nitrobenzoic acid degradation pathway of Pseudomonas fluorescens strain KU-7 proceeds via a novel 3-hydroxyanthranilate intermediate. In this study, we cloned and sequenced a 19-kb DNA locus of strain KU-7 that encompasses the 3-hydroxyanthranilate meta-cleavage pathway genes. The gene cluster, designated nbaEXHJIGFCDR, is organized tightly and in the same direction. The nbaC and nbaD gene products were found to be novel homologs of the eukaryotic 3-hydroxyanthranilate 3,4-dioxygenase and 2-amino-3-carboxymuconate-6-semialdehyde decarboxylase, respectively. The NbaC enzyme carries out the oxidation of 3-hydroxyanthranilate to 2-amino-3-carboxymuconate-6-semialdehyde, while the NbaD enzyme catalyzes the decarboxylation of the latter compound to 2-aminomuconate-6-semialdehyde. The NbaC and NbaD proteins were overexpressed in Escherichia coli and characterized. The substrate specificity of the 23.8-kDa NbaC protein was found to be restricted to 3-hydroxyanthranilate. In E. coli, this enzyme oxidizes 3-hydroxyanthranilate with a specific activity of 8 U/mg of protein. Site-directed mutagenesis experiments revealed the essential role of two conserved histidine residues (His52 and His96) in the NbaC sequence. The NbaC activity is also dependent on the presence of Fe(2+) but is inhibited by other metal ions, such as Zn(2+), Cu(2+), and Cd(2+). The NbaD protein was overproduced as a 38.7-kDa protein, and its specific activity towards 2-amino-3-carboxymuconate-6-semialdehyde was 195 U/mg of protein. Further processing of 2-aminomuconate-6-semialdehyde to pyruvic acid and acetyl coenzyme A was predicted to proceed via the activities of NbaE, NbaF, NbaG, NbaH, NbaI, and NbaJ. The predicted amino acid sequences of these proteins are highly homologous to those of the corresponding proteins involved in the metabolism of 2-aminophenol (e.g., AmnCDEFGH in Pseudomonas sp. strain AP-3). The NbaR-encoding gene is predicted to have a regulatory function of the LysR family type. The function of the product of the small open reading frame, NbaX, like the homologous sequences in the nitrobenzene or 2-aminophenol metabolic pathway, remains elusive.     

Anthranilic acid metabolism by microorganisms. Formation of 5-hydroxyanthranilate as an intermediate in anthranilate metabolism byNocardia opaca

5-Hydroxyanthranilic acid was isolated and characterized as an intermediate in the metabolism of anthranilate byNocardia opaca. This compound appeared during the log phase of growth and led to the subsequent induction of high levels of enzymes for its subsequent degradation. Gentisate was also found as a product of 5-hydroxyanthranilate metabolism. Both 5-hydroxyanthranilate and gentisate were oxidized to pyruvate by extracts of anthranilate-grown cells, the ring cleavage of the diphenol being effected by an oxygenase with a typical Fe²⁺ requirement. Features of the metabolism of 5-hydroxyanthranilate and gentisate byN. opaca, which can also oxidize anthranilate through catechol, suggest that the gentisate pathway is a secondary route.     

Characterization of a pseudomonad 2-nitrobenzoate nitroreductase and its catabolic pathway-associated 2-hydroxylaminobenzoate mutase and a chemoreceptor involved in 2-nitrobenzoate chemotaxis

Pseudomonas fluorescens strain KU-7 is a prototype microorganism that metabolizes 2-nitrobenzoate (2-NBA) via the formation of 3-hydroxyanthranilate (3-HAA), a known antioxidant and reductant. The initial two steps leading to the sequential formation of 2-hydroxy/aminobenzoate and 3-HAA are catalyzed by a NADPH-dependent 2-NBA nitroreductase (NbaA) and 2-hydroxylaminobenzoate mutase (NbaB), respectively. The 216-amino-acid protein NbaA is 78% identical to a plasmid-encoded hypothetical conserved protein of Polaromonas strain JS666; structurally, it belongs to the homodimeric NADH:flavin mononucleotide (FMN) oxidoreductase-like fold family. Structural modeling of complexes with the flavin, coenzyme, and substrate suggested specific residues contributing to the NbaA catalytic activity, assuming a ping-pong reaction mechanism. Mutational analysis supports the roles of Asn40, Asp76, and Glu113, which are predicted to form the binding site for a divalent metal ion implicated in FMN binding, and a role in NADPH binding for the 10-residue insertion in the beta5-alpha2 loop. The 181-amino-acid sequence of NbaB is 35% identical to the 4-hydroxylaminobenzoate lyases (PnbBs) of various 4-nitrobenzoate-assimilating bacteria, e.g., Pseudomonas putida strain TW3. Coexpression of nbaB with nbaA in Escherichia coli produced a small amount of 3-HAA from 2-NBA, supporting the functionality of the nbaB gene. We also showed by gene knockout and chemotaxis assays that nbaY, a chemoreceptor NahY homolog located downstream of the nbaA gene, is responsible for strain KU-7 being attracted to 2-NBA. NbaY is the first chemoreceptor in nitroaromatic metabolism to be identified, and this study completes the gene elucidation of 2-NBA metabolism that is localized within a 24-kb chromosomal locus of strain KU-7.     

Regulation of the Pathway for the Degradation of Anthranilate in Aspergillus niger

Studies were carried out to determine the factors governing the induction of anthranilate hydroxylase and other enzymes in the pathway for the dissimilation of anthranilate by Aspergillus niger (UBC 814). The enzyme was induced by growth in the presence of tryptophan, kynurenine, anthranilate, and, surprisingly, by 3-hydroxyanthranilate, which was not an intermediate in the conversion of anthranilate to 2,3-dihydroxybenzoate. There was an initial lag in the synthesis of anthranilate hydroxylase when induced by tryptophan, anthranilate, and 3-hydroxyanthranilate. Cycloheximide inhibited the enzyme induction. Comparative studies on anthranilate hydroxylase, 2,3-dihydroxybenzoate carboxy-lyase, and catechol 1:2-oxygenase revealed that these enzymes were not coordinately induced by either anthranilate or 3-hydroxyanthranilate. Structural requirements for the induction of anthranilate hydroxylase were determined by using various analogues of anthranilate. The activity of the constitutive catechol oxygenase was increased threefold by exposure to anthranilate, 2,3-dihydroxybenzoate, or catechol. 3-Hydroxyanthranilate did not enhance the levels of catechol oxygenase activity.     

Biodegradation of 3-Nitrobenzoate by <Emphasis Type="Italic">Bacillus flexus</Emphasis> strain XJU-4

Bacillus flexus strain XJU-4 utilized 3-nitrobenzoate at 12 mM as a sole source of carbon and energy. This strain also utilized 4-nitrobenzoate, 2-nitrotoluene and nitrobenzene as growth substrates. The optimum conditions for degradation of 3-nitrobenzoate by the organism were found to be at pH 7.0 and temperature 30°C. Metabolite analysis, growth and enzymatic studies have revealed that the organism degraded 3-nitrobenzoate by oxidative mechanism through protocatechuate with the release of nitrite. The cells grown on 3-nitrobenzoate utilized protocatechuate but not 3-hydroxybenzoate, 3-aminobenzoate, 4-hydroxy-3-nitrobenzoate and 4-nitrocatechol. The cell-free extract of Bacillus flexus strain XJU-4 grown on 3-nitrobenzoate contained the activity of protocatechuate 2,3-dioxygenase, which suggest that protocatechuate was further degraded by a novel 2,3-dioxygenative meta-cleavage pathway.     

Nitrobenzoates and aminobenzoates are chemoattractants for Pseudomonas strains

Three Pseudomonas strains were tested for the ability to sense and respond to nitrobenzoate and aminobenzoate isomers in chemotaxis assays. Pseudomonas putida PRS2000, a strain that grows on benzoate and 4-hydroxybenzoate by using the beta-ketoadipate pathway, has a well-characterized beta-ketoadipate-inducible chemotactic response to aromatic acids. PRS2000 was chemotactic to 3- and 4-nitrobenzoate and all three isomers of aminobenzoate when grown under conditions that induce the benzoate chemotactic response. P. putida TW3 and Pseudomonas sp. strain 4NT grow on 4-nitrotoluene and 4-nitrobenzoate by using the ortho (beta-ketoadipate) and meta pathways, respectively, to complete the degradation of protocatechuate derived from 4-nitrotoluene and 4-nitrobenzoate. However, based on results of catechol 1,2-dioxygenase and catechol 2,3-dioxygenase assays, both strains were found to use the beta-ketoadipate pathway for the degradation of benzoate. Both strains were chemotactic to benzoate, 3- and 4-nitrobenzoate, and all three aminobenzoate isomers after growth with benzoate but not succinate. Strain TW3 was chemotactic to the same set of aromatic compounds after growth with 4-nitrotoluene or 4-nitrobenzoate. In contrast, strain 4NT did not respond to any aromatic acids when grown with 4-nitrotoluene or 4-nitrobenzoate, apparently because these substrates are not metabolized to the inducer (beta-ketoadipate) of the chemotaxis system. The results suggest that strains TW3 and 4NT have a beta-ketoadipate-inducible chemotaxis system that responds to a wide range of aromatic acids and is quite similar to that present in PRS2000. The broad specificity of this chemotaxis system works as an advantage in strains TW3 and 4NT because it functions to detect diverse carbon sources, including 4-nitrobenzoate.     

Nitrobenzoates and Aminobenzoates Are Chemoattractants for Pseudomonas Strains

Three Pseudomonas strains were tested for the ability to sense and respond to nitrobenzoate and aminobenzoate isomers in chemotaxis assays. Pseudomonas putida PRS2000, a strain that grows on benzoate and 4-hydroxybenzoate by using the β-ketoadipate pathway, has a well-characterized β-ketoadipate-inducible chemotactic response to aromatic acids. PRS2000 was chemotactic to 3- and 4-nitrobenzoate and all three isomers of aminobenzoate when grown under conditions that induce the benzoate chemotactic response. P. putida TW3 and Pseudomonas sp. strain 4NT grow on 4-nitrotoluene and 4-nitrobenzoate by using the ortho (β-ketoadipate) and meta pathways, respectively, to complete the degradation of protocatechuate derived from 4-nitrotoluene and 4-nitrobenzoate. However, based on results of catechol 1,2-dioxygenase and catechol 2,3-dioxygenase assays, both strains were found to use the β-ketoadipate pathway for the degradation of benzoate. Both strains were chemotactic to benzoate, 3- and 4-nitrobenzoate, and all three aminobenzoate isomers after growth with benzoate but not succinate. Strain TW3 was chemotactic to the same set of aromatic compounds after growth with 4-nitrotoluene or 4-nitrobenzoate. In contrast, strain 4NT did not respond to any aromatic acids when grown with 4-nitrotoluene or 4-nitrobenzoate, apparently because these substrates are not metabolized to the inducer (β-ketoadipate) of the chemotaxis system. The results suggest that strains TW3 and 4NT have a β-ketoadipate-inducible chemotaxis system that responds to a wide range of aromatic acids and is quite similar to that present in PRS2000. The broad specificity of this chemotaxis system works as an advantage in strains TW3 and 4NT because it functions to detect diverse carbon sources, including 4-nitrobenzoate.     

Evidence for distinct kynureninase and hydroxykynureninase activities in Neurospora crassa

Previous studies have indicated that a single enzyme, "kynureninase," catalyzes the reactions of l-kynurenine to anthranilate and l-3-hydroxykynurenine to 3-hydroxyanthranilate in Neurospora crassa and in other organisms. The present report describes separate enzymes which catalyze these reactions in N. crassa. The first, a kynureninase, preferentially catalyzes kynurenine to anthranilate and is induced over 400-fold by tryptophan or a catabolite of tryptophan. The second, a hydroxykynureninase, is constitutive or noninducible by tryptophan and preferentially catalyzes l-3-hydroxykynurenine to 3-hydroxyanthranilate. The physiological significance of these enzymes may be inferred from the facts that (i) the noninducible enzyme hydroxykynureninase appears to be the main enzyme present in uninduced cells that is capable of catalyzing l-3-hydroxykynurenine to 3-hydroxyanthranilate for the indispensible synthesis of nicotinamide adenine dinucleotide, and (ii) the inducible enzyme kynureninase is induced by tryptophan to a concentration far in excess of that needed to meet the requirements of the cells for nicotinamide adenine dinucleotide, resulting in the excretion of anthranilate into the medium.     

Chemotaxis of <Emphasis Type="Italic">Burkholderia</Emphasis> sp. Strain SJ98 towards chloronitroaromatic compounds that it can metabolise

Background  

Burkholderia sp. strain SJ98 is known for its chemotaxis towards nitroaromatic compounds (NACs) that are either utilized as sole sources of carbon and energy or co-metabolized in the presence of alternative carbon sources. Here we test for the chemotaxis of this strain towards six chloro-nitroaromatic compounds (CNACs), namely 2-chloro-4-nitrophenol (2C4NP), 2-chloro-3-nitrophenol (2C3NP), 4-chloro-2-nitrophenol (4C2NP), 2-chloro-4-nitrobenzoate (2C4NB), 4-chloro-2-nitrobenzoate (4C2NB) and 5-chloro-2-nitrobenzoate (5C2NB), and examine its relationship to the degradation of such compounds.     

Complete nucleotide sequence of the 113-kilobase linear catabolic plasmid pAL1 of Arthrobacter nitroguajacolicus Rü61a and transcriptional analysis of genes involved in quinaldine degradation

The nucleotide sequence of the linear catabolic plasmid pAL1 from the 2-methylquinoline (quinaldine)-degrading strain Arthrobacter nitroguajacolicus Rü61a comprises 112,992 bp. A total of 103 open reading frames (ORFs) were identified on pAL1, 49 of which had no annotatable function. The ORFs were assigned to the following functional groups: (i) catabolism of quinaldine and anthranilate, (ii) conjugation, and (iii) plasmid maintenance and DNA replication and repair. The genes for conversion of quinaldine to anthranilate are organized in two operons that include ORFs presumed to code for proteins involved in assembly of the quinaldine-4-oxidase holoenzyme, namely, a MobA-like putative molybdopterin cytosine dinucleotide synthase and an XdhC-like protein that could be required for insertion of the molybdenum cofactor. Genes possibly coding for enzymes involved in anthranilate degradation via 2-aminobenzoyl coenzyme A form another operon. These operons were expressed when cells were grown on quinaldine or on aromatic compounds downstream in the catabolic pathway. Single-stranded 3' overhangs of putative replication intermediates of pAL1 were predicted to form elaborate secondary structures due to palindromic and superpalindromic terminal sequences; however, the two telomeres appear to form different structures. Sequence analysis of ORFs 101 to 103 suggested that pAL1 codes for one or two putative terminal proteins, presumed to be covalently bound to the 5' termini, and a multidomain telomere-associated protein (Tap) comprising 1,707 amino acids. Even if the putative proteins encoded by ORFs 101 to 103 share motifs with the Tap and terminal proteins involved in telomere patching of Streptomyces linear replicons, their overall sequences and domain structures differ significantly.     

The utilization of some halogenated aromatic acids by Nocardia. Oxidation and metabolism

1. Halogen analogues of p-nitrobenzoate and benzoate were oxidized by washed cells of Nocardia erythropolis. 2. The oxidation of 2-fluoro-4-nitrobenzoate ceased at the level of acetate, and fluoroacetate was found in the incubation medium and particularly in hot-ethanolic extracts of the cells. 3. Several fluorine-containing intermediates were detected and 2-fluoroprotocatechuate was identified as one of them. 4. The nitro group was also reduced by the organism, as evidenced by the formation of 4-amino-2-fluorobenzoate. 5. Extracts of N. erythropolis activated fluoroacetate and condensed the resulting fluoroacetyl-CoA with oxaloacetate to form fluorocitrate. This product was a very powerful inhibitor of citrate metabolism by guinea-pig kidney homogenates and of the aconitase also present in the bacterial extracts. The inhibitions effected by synthetic fluorocitrate and the natural product were comparable. 6. 2-Fluoro-4-nitrobenzoate had negligible mammalian toxicity. 7. The isolation of fluoroacetate as a product of 2-fluoro-4-nitrobenzoate oxidation implies that the aromatic ring in this bacterium must be degraded via a gamma-carboxymuconolactone; fluoroacetate cannot arise by metabolism through the isomeric beta-carboxymuconolactone.     

Proteomics analysis of aromatic catabolic pathways in thermophilic Geobacillus thermodenitrificans NG80-2

Geobacillus thermodenitrificans NG80-2 is a crude oil-degrading thermophilic bacterium isolated from an oil reservoir in China. In this study, the gene clusters and pathways for the degradation of benzoate (via benzoyl-CoA), phenylacetate (via phenylacetyl-CoA), 4-hydroxyphenylacetate (via 3,4-dihydroxyphenylacetate) and anthranilate (via 3-hydroxyanthranilate) were confirmed using combined in silico analysis and proteomics approaches. It was observed that synthesis of the enzymes catalyzing initial activation, ring oxidation and ring cleavage reactions were generally induced specifically by their respective substrates, while many of the enzymes catalyzing downstream reactions exhibited broader substrate specificities. Novel genes encoding benzoyl-CoA epoxidase and 3,4-dihydroxyphenylacetate 2,3-dioxygenase, and a paaX homologue that serves as a positive regulator of benzoate degradation were proposed. Downregulation of the glycolysis pathway, along with upregulation of the gluconeogenesis pathway and the glyoxylate bypass (phenylacetate) were detected in association with the utilization of the aromatics. This novel proteomics approach confirmed the presence of multiple metabolic pathways for aromatic compounds in NG80-2, which is highly advantageous to the survival of this thermophilic bacterium under reservoir conditions.     

Biomineralization of 3-nitrotoluene by Diaphorobacter species

Three bacterial strains utilizing 3-nitrotoluene (3-NT) as a sole source of carbon, nitrogen and energy were isolated from an industrial wastewater treatment plant. Biochemical tests and 16S rDNA sequence analysis revealed that the isolated strains belonged to Diaphorobacter sp. Detailed studies were carried out with Diaphorobacter sp. strain DS2. Degradation of 3-NT by Diaphorobacter sp. strain DS2 was accompanied by the release of nitrite in the culture broth with increase in biomass. Total organic carbon analysis confirmed the extensive mineralization of 3-NT. The strain could degrade 3-methylcatechol, 4-methylcatechol and catechol easily suggesting that the degradation pathway could involve these as possible intermediates. Successful PCR amplification of the oxygenase large subunit and the presence of high activity for catechol 2,3-dioxygenase in the crude cell lysate further confirmed that the degradation of 3-NT occurred through (methyl)catechol intermediates in strain DS2. The strain DS2 was found to degrade other isomers of mononitrotoluene (2-NT and 4-NT) and nitrobenzene as well.     

The biosynthesis of phytoalexins in Dianthus caryophyllus L. cell cultures: induction of benzoyl-CoA:anthranilate N-benzoyltransferase activity

It has been shown that cell cultures of Dianthus caryophyllus L. c.v. Eleganz accumulate N-benzoyl-4-methoxyanthranilic acid, previously identified as the phytoalexin methoxydianthramide B, in response to treatment either with a crude elicitor isolated from the cell walls of Phytophthora megasperma f.sp. glycinea or with a commercial yeast extract. Cell-free extracts from the induced cells efficiently catalyzed the N-benzoylation of anthranilate in the presence of benzoyl-CoA. The partially purified transferase was shown to be specific for anthranilate with almost no activity toward 4-hydroxyanthranilate, whereas acyl donors other than benzoyl-CoA such as salicyloyl-, cinnamoyl-, or 4-coumaroyl-CoA were also accepted. Elicitor treatment of the cells additionally induced an S-adenosyl-L-methionine:N-benzoyl-4-hydroxyanthranilate 4-O-methyltransferase activity. We propose, therefore, that methoxydianthramide B is derived from N-benzoylanthranilic acid via N-benzoyl-4-hydroxyanthranilic acid. Dark-grown cells contained little N-benzoyltransferase activity (approx 8 mu kat/kg), which increased roughly ninefold within 6 h following the addition of the elicitor. In addition, phenylalanine ammonia-lyase activity of the cells increased about twofold under these conditions to a maximum (approx 40 mu kat/kg) at 5 h. The rapid induction of both enzyme activities suggests that the shikimate pathway is of crucial importance in the disease resistance response of carnation cells.     

微生物降解对氯苯胺的一条新代谢途径

迄今为止的研究报道表明,对氯苯胺的生物降解只能以邻位途径或修饰邻位途径进行。采用HPLC、液相色谱质谱联用技术(LC/MS)对Diaphorobacter PCA039菌株降解对氯苯胺的中间代谢产物进行了分析和鉴定,结果表明,对氯苯胺经PCA039菌株的降解形成了氯代邻苯二酚,5-氯-4草酰巴豆酸,5-氯-2-氧戊烯酸,5-氯-2-氧-4-羟戊酸,氯代乙酸等中间代谢产物,这些都是典型的间位代谢途径(meta-pathway)的中间物质,说明Diaphorobacter PCA039菌株以间位裂解途径对对氯苯胺进行降解。这对于对氯代胺的生物降解代谢研究、代谢机理及其遗传表达调控研究具有意义。     

Characterization of Fluoroglycofen Ethyl Degradation by Strain <Emphasis Type="Italic">Mycobacterium phocaicum</Emphasis> MBWY-1

A fluoroglycofen ethyl-degrading bacterium, MBWY-1, was isolated from the soil of an herbicide factory. This isolated strain was identified as Mycobacterium phocaicum based on analysis of its 16S rRNA gene sequence and its morphological, physiological, and biochemical properties. The strain was able to utilize fluoroglycofen ethyl as its sole source of carbon for growth and could degrade 100 mg l⁻¹ of fluoroglycofen ethyl to a non-detectable level within 72 h. The optimum temperature and pH for fluoroglycofen ethyl degradation by strain MBWY-1 were 30°C and 7.0, respectively. Five metabolites produced during the degradation of fluoroglycofen ethyl and were identified by mass spectrometry as {5-[2-chloro-4-(trifluoromethyl) phenoxy]-2-nitrophenylacyl} hydroxyacetic acid, acifluorfen, 5-[2-chloro-4-(trifluoromethyl) phenoxy]-2-nitrobenzoate, 5-[2-chloro-4-(trifluoromethyl) phenoxy]-2-hydroxyl, and 3-chloro-4-hydroxyl benzotrifluoride. Identification of the metabolites allowed to propose the degradation pathway of fluoroglycofen ethyl by strain MBWY-1. The inoculation of strain MBWY-1 into soil treated with fluoroglycofen ethyl resulted in a higher fluoroglycofen ethyl degradation rate than in uninoculated soil regardless of whether the soil was sterilized or nonsterilized.     

Continuous degradation of mixtures of 4-nitrobenzoate and 4-aminobenzoate by immobilized cells of Burkholderia cepacia strain PB4

 Although isolated on 4-aminobenzoate, Burkholderia cepacia strain PB4 is also able to grow on 4-nitrobenzoate. Degradation of an equimolar mixture of the nitroaromatic compound 4-nitrobenzoate and its corresponding aminoaromatic derivative 4-aminobenzoate by this strain was investigated. Batch experiments showed that, irrespective of preculturing conditions, both compounds were degraded simultaneously. The mixture-degrading ability of B. cepacia strain PB4 was subsequently tested in continuous packed bed reactors (PBR) with the strain immobilized on Celite grade R-633 or R-635. Higher degradation rates were achieved with the larger particles of Celite R-635. Maximum simultaneous degradation rates per liter of packed bed of 0.925 mmol l⁻¹ h⁻¹ 4-nitrobenzoate and 4-aminobenzoate were obtained for an applied loading rate of the same value (0.925 mmol l⁻¹ h⁻¹ of each compound). Even when the applied load was not removed in its entirety, neither of the two compounds was degraded preferentially but a percentage of both of them was mineralized. The present study shows the possibility for a pure strain to biodegrade not only a nitroaromatic compound (4-nitrobenzoate) but also its corresponding amino derivative (4-aminobenzoate) continuously and simultaneously. Received: 23 November 1998 / Revision received: 6 April 1999 / Accepted: 9 April 1999