A genetic analysis system of Burkholderia cepacia: construction of mobilizable transposons and a cloning vector

Lipolytic activity of suspended and membrane immobilized lipase originating from indigenous Burkholderia sp. C20

In this work, a simple, inexpensive, and efficient method of preparing immobilized lipase is presented. The lipase originating from a newly isolated indigenous strain Burkholderia sp. C20 was immobilized onto cellulose nitrate (CN) membrane via filtration. The CN-immobilized lipase was able to retain 60% of its original activity after repeated uses for nine times. The thermal stability of the lipase was also slightly improved after immobilization. The optimal reaction conditions of CN-lipase were pH 9.0 and 55 degrees C, which are similar to those for the suspended lipase. Both suspended and immobilized lipase could hydrolyze the six oil substrates examined, while immobilized lipase displayed less specificity over the oil substrates. Kinetic analysis shows that the dependence of lipolytic activity of both suspended and immobilized lipase on oil substrate concentration can be described by Michaelis-Menten model with good agreement. The estimated kinetic constants for suspended lipase (v(max)=243.9 U/mg, K(m)=0.024 mM) and immobilized lipase (v(max)=32.8 U/mg, K(m)=5.61 mM) were quite different. Employment of immobilization seemed to result in a decrease in v(max) and an increase in K(m), most likely due to the mass transfer resistance arising from formation of micelles during the lipase immobilization process.

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.

Tn5530 from Burkholderia cepacia strain 2a encodes a chloride channel protein essential for the catabolism of 2,4-dichlorophenoxyacetic acid

Chloride channel proteins (ClC) are found in living systems where they transport chloride ions across cell membranes. Recently, the structure/function of two prokaryotic ClC has been determined but little is known about the role of these proteins in the microbial metabolism of chlorinated compounds. Here we show that transposon Tn5530 from Burkholderia cepacia strain 2a encodes a ClC protein (BcClC) which is responsible for expelling Cl(-) ions generated during the catabolism of 2,4-dichlorophenoxyacetic acid (a chlorinated herbicide). We found that BcClC has the ability to transport Cl(-) ions across reconstituted proteoliposome membranes. We created two mutants in which the intrachannel glutamate residue of the protein, known to be responsible for opening and closing the channel (i.e. gating), was changed in order to create constitutively open and closed forms. We observed that cells carrying the closed-channel protein accumulated Cl(-) ions intracellularly leading to a decrease in intracellular pH, cell stasis and death. Further, we established that BcClC has the same gating mechanism as that reported for the ClC protein from Salmonella typhimurium. Our results show that the physiological role of ClC is to maintain cellular homeostasis which can be impaired by the catabolism of chlorinated compounds.

Isolation and characterization of Burkholderia cenocepacia mutants deficient in pyochelin production: pyochelin biosynthesis is sensitive to sulfur availability

The opportunistic pathogen Burkholderia cenocepacia produces the yellow-green fluorescent siderophore, pyochelin. To isolate mutants which do not produce this siderophore, we mutagenized B. cenocepacia with the transposon mini-Tn5Tp. Two nonfluorescent mutants were identified which were unable to produce pyochelin. In both mutants, the transposon had integrated into a gene encoding an orthologue of CysW, a component of the sulfate/thiosulfate transporter. The cysW gene was located within a putative operon encoding other components of the transporter and a polypeptide exhibiting high homology to the LysR-type regulators CysB and Cbl. Sulfate uptake assays confirmed that both mutants were defective in sulfate transport. Growth in the presence of cysteine, but not methionine, restored the ability of the mutants to produce pyochelin, suggesting that the failure to produce the siderophore was the result of a depleted intracellular pool of cysteine, a biosynthetic precursor of pyochelin. Consistent with this, the wild-type strain did not produce pyochelin when grown in the presence of lower concentrations of sulfate that still supported efficient growth. We also showed that whereas methionine and certain organosulfonates can serve as sole sulfur sources for this bacterium, they do not facilitate pyochelin biosynthesis. These observations suggest that, under conditions of sulfur depletion, cysteine cannot be spared for production of pyochelin even under iron starvation conditions.

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.

Construction and evaluation of plasmid vectors optimized for constitutive and regulated gene expression in Burkholderia cepacia complex isolates

Genetic studies with Burkholderia cepacia complex isolates are hampered by the limited availability of cloning vectors and by the inherent resistance of these isolates to the most common antibiotics used for genetic selection. Also, some of the promoters widely employed for gene expression in Escherichia coli are inefficient in B. cepacia. In this study, we have utilized the backbone of the vector pME6000, a derivative of the pBBR1 plasmid that was originally isolated from Bordetella bronchiseptica, to construct a set of vectors useful for gene expression in B. cepacia. These vectors contain either the constitutive promoter of the S7 ribosomal protein gene from Burkholderia sp. strain LB400 or the arabinose-inducible P(BAD) promoter from E. coli. Promoter sequences were placed immediately upstream of multiple cloning sites in combination with the minimal sequence of pME6000 required for plasmid maintenance and mobilization. The functionality of both vectors was assessed by cloning the enhanced green fluorescent protein gene (e-gfp) and determining the levels of enhanced green fluorescent protein expression and fluorescence emission for a variety of clinical and environmental isolates of the B. cepacia complex. We also demonstrate that B. cepacia carrying these constructs can readily be detected intracellularly by fluorescence microscopy following the infection of Acanthamoeba polyphaga.

The dsbA-dsbB disulfide bond formation system of Burkholderia cepacia is involved in the production of protease and alkaline phosphatase, motility, metal resistance, and multi-drug resistance

In a previous study, we isolated a dsbB mutant of Burkholderia cepacia KF1 and showed that phenotypes of protease production and motility are dependent on DsbB, a membrane-bound disulfide bond oxidoreductase. We have now isolated a dsbA mutant by transposon mutagenesis, cloned the dsbA gene encoding a periplasmic disulfide bond oxidoreductase, and characterized the function of the DsbA-DsbB disulfide bond formation system in B. cepacia. The complementing DNA fragment had an open reading frame for a 212-amino acid polypeptide with a potential redox-active site sequence of Cys-Pro-His-Cys that is homologous to Escherichia coli DsbA. The dsbA mutant, as well as the previously isolated dsbB mutant, was defective in the production of extracellular protease and alkaline phosphatase, as well as in motility. In addition, mutation in the DsbA-DsbB system resulted in an increase in sensitivity to Cd2+ and Zn2+ as well as a variety of antibiotics including beta-lactams, kanamycin, erythromycin, novobiocin, ofloxacin and sodium dodecyl sulfate. These results suggested that the DsbA-DsbB system might be involved in the formation of a metal efflux system as well as a multi-drug resistance system.

pTn5cat: a Tn5-derived genetic element to facilitate insertion mutagenesis, promoter probing, physical mapping, cloning, and marker exchange in phytopathogenic and other gram-negative bacteria

A Tn5-derived mobile element has been constructed to identify genes and promoters related to pathogenesis and virulence in Pseudomonas syringae pv. phaseolicola. To enhance the rate of mutation this Tn5 derivative was constructed carrying a mutant transposase which was placed in cis to the transposable element, but just outside the inverted repeats, therefore eliminating secondary transposition and increasing the stability of the insertion. The new element also contains a promoterless cat (chloramphenicol acetyltransferase) gene as reporter to allow for positive selection of promoters being expressed under specific conditions. To facilitate cloning and manipulations in Escherichia coli, a ColE1 origin of replication has been included within the transposable element as well as the Mob region from the broad-host-range plasmid RP4, which allows this element to be efficiently mobilized by a triparental mating or by using an E. coli strain such as S17-1 to provide the tra functions. Sites for the rare cutters PacI and PmeI have also been included to facilitate locating the insertions on a PacI and/or PmeI physical map. This construction combines the properties of both a mobilizable plasmid and a transposon and therefore has been termed pTn5cat. It is almost the same size as the wild-type Tn5, 5877 bp, and has successfully been tested in P.s. phaseolicola and Xanthomonas campestris pv. campestris.