Characterization of the 450-kb linear plasmid in a polychlorinated biphenyl degrader, Rhodococcus sp. strain RHA1

A strong polychlorinated biphenyl (PCB) degrader, Rhodococcus sp. strain RHA1, has diverse biphenyl/PCB degradative genes and harbors huge linear plasmids, including pRHL1 (1,100 kb), pRHL2 (450 kb), and pRHL3 (330 kb). The diverse degradative genes are distributed mainly on the pRHL1 and pRHL2 plasmids. In this study, the structural and functional characteristics of pRHL2 were determined. We constructed a physical map of pRHL2, and the degradative enzyme genes, including bphB2, etbD2, etbC, bphDEF, bphC2, and bphC4, were localized in three regions. Conjugal transfer of pRHL2 between RHA1 mutant derivatives was observed at a frequency of 7.5 x 10(-5) transconjugant per recipient. These results suggested that the linear plasmid is a possible determinant of propagation of the diverse degradative genes in rhodococci. The termini of pRHL2 were cloned and sequenced. The left and right termini of pRHL2 had 3-bp perfect terminal inverted repeats and were not as similar to each other (64% identity) as the known actinomycete linear replicons are. Southern hybridization analysis with pRHL2 terminal probes suggested that the right terminus of pRHL2 is similar to pRHL1 and pRHL3 termini. Retardation of both terminal fragments in the gel shift assay indicated that each terminus of pRHL2 is linked to a protein. We suggest that pRHL2 has invertron termini, as has been reported previously for Streptomyces linear replicons.     

Expression in Escherichia coli of biphenyl 2,3-dioxygenase genes from a Gram-positive polychlorinated biphenyl degrader, Rhodococcus jostii RHA1

Rhodococcus jostii RHA1 is a polychlorinated biphenyl degrader. Multi-component biphenyl 2,3-dioxygenase (BphA) genes of RHA1 encode large and small subunits of oxygenase component and ferredoxin and reductase components. They did not express enzyme activity in Escherichia coli. To obtain BphA activity in E. coli, hybrid BphA gene derivatives were constructed by replacing ferredoxin and/or reductase component genes of RHA1 with those of Pseudomonas pseudoalcaligenes KF707. The results obtained indicate a lack of catalytic activity of the RHA1 ferredoxin component gene, bphAc in E. coli. To determine the cause of inability of RHA1 bphAc to express in E. coli, the bphAc gene was introduced into Rosetta (DE3) pLacI, which has extra tRNA genes for rare codons in E. coli. The resulting strain abundantly produced the bphAc product, and showed activity. These results suggest that codon usage bias is involved in inability of RHA1 bphAc to express its catalytic activity in E. coli.     

Characterization of two biphenyl dioxygenases for biphenyl/PCB degradation in A PCB degrader, Rhodococcus sp. strain RHA1

Rhodococcus sp. RHA1 induces two biphenyl dioxygenases, the BphA and EtbA/EbdA dioxygenases, during growth on biphenyl. Their subunit genes were expressed in R. erythropolis IAM1399 to investigate the involvement of each subunit gene in their activity and their substrate preferences. The recombinant expressing ebdA1A2A3etbA4 and that expressing bphA1A2A3A4 exhibited 4-chlorobiphenyl (4-CB) transformation activity, suggesting that these gene sets are responsible for the EtbA/EbdA and BphA dioxygenases respectively. When bphA4 and etbA4 were swapped to construct the recombinants expressing ebdA1A2A3bphA4 and bphA1A2A3etbA4 respectively, compatibility between BphA4 and EtbA4 was suggested by their 4-CB transformation activities. When bphA3 and ebdA3 were swapped, incompatibility between BphA3 and EbdA3 was suggested. BphA and EtbA/EbdA dioxygenases exhibited the highest transformation activity toward biphenyl and naphthalene respectively, and also attacked dibenzofuran and dibenzo-p-dioxin. The wide substrate preference of EtbA/EbdA dioxygenase suggested that it plays a more important role in polychlorinated biphenyl (PCB) degradation than does BphA dioxygenase.     

The bphDEF meta-cleavage pathway genes involved in biphenyl/polychlorinated biphenyl degradation are located on a linear plasmid and separated from the initial bphACB genes in Rhodococcus sp. strain RHA1

The bphACB genes responsible for the initial oxidation of the aromatic ring of biphenyl/polychlorinated biphenyls (PCB) to meta-cleavage product in Rhodococcus sp. RHA1 have been characterized. We cloned the 6.1 kb EcoRI fragment containing another extradiol dioxygenase gene (etbC) which was induced during the growth on ethylbenzene. The bphD, bphE and bphF encoding 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoate (HOPD) hydrolase, 2-hydroxypenta-2,4-dienoate hydratase and 4-hydroxy-2-oxovalerate aldolase, respectively, were found downstream of etbC. The deduced amino acid (aa) sequence of RHA1 bphD and bphE had 27–33% and 32–38% identity, respectively, with those of the corresponding genes in Pseudomonas. BphE and BphF are closely related to the corresponding homoprotocatechuate meta-cleavage pathway enzymes of Escherichia coli C. The bphD and bphF were expressed in E. coli and the BphD activity was detected. The etbCbphDEF genes were transcribed in biphenyl and ethylbenzene growing cells. Pulsed field gel electrophoresis (PFGE) analysis indicated that RHA1 contains three large linear plasmids. Southern blot analysis indicated that the meta-cleavage pathway for biphenyl/PCB catabolism in RHA1 is directed by the 390 kb plasmid borne bphDEF genes located separately from bphACB gene cluster on the 1100 kb plasmid.     

Multiplicity of aromatic ring hydroxylation dioxygenase genes in a strong PCB degrader, Rhodococcus sp. strain RHA1 demonstrated by denaturing gradient gel electrophoresis

To address the multiplicity of aromatic ring hydroxylation dioxygenases, we used PCR amplification and denaturing gradient gel electrophoresis (DGGE). The amplified DNA fragments separated into five bands, A to E. Southern hybridization analysis of RHA1 total DNA using the probes for each band showed that band C originated from a couple of homologous genes. The nucleotide sequences of the bands showed that bands A, C, and E would be parts of new dioxygenase genes in RHA1. That of band B agreed with the bphA1 gene, which was characterized previously. That of band D did not correspond to any known gene sequences. The regions including the entire open reading frames (ORFs) were cloned and sequenced. The nucleotide sequences of ORFs suggested that the genes of bands A, C, and E may respectively encode benzoate, biphenyl, and polyhydrocarbon dioxygenases. Northern hybridization indicated the induction of the gene of band A by benzoate and biphenyl, and that of the gene of band C by biphenyl and ethylbenzene, supporting the above notions. The gene of band E was not induced by any of these substrates. Thus the combination of DGGE and Southern hybridization enable us to address the multiplicity of the ring hydroxylation dioxygenase genes and to isolate some of them.     

Structural alteration of linear plasmids encoding the genes for polychlorinated biphenyl degradation in Rhodococcus strain RHA1

Polychlorinated biphenyl (PCB) tolerant derivatives of a strong PCB degrader, Rhodococcus strain RHA1, were selected after growth in the presence of 100 g/ml PCBs. Some of the derivatives did not grow on biphenyl but accumulated a yellow coloured metabolite suggesting a defect in the meta-ring-cleavage compound hydrolase step encoded by the bphD gene. Other derivatives failed to grow on biphenyl and exhibited little PCB transformation activity suggesting a defect in the initial ring-hydroxylation dioxygenase step encoded by the bphA gene. These organisms had a structural alteration in the linear plasmids coding for the bph genes in RHA1, which included the bph gene deletion. When a bphD containing plasmid was introduced into a tolerant derivative, RCD1, which was shown to have a bphD deletion, the defect in the growth on biphenyl of RCD1 was overcome. The bph gene deletion seems to play a key role in these tolerant derivatives thereby suggesting that the toxic metabolic intermediate would be a main cause of the growth inhibition of RHA1 in the presence of high concentration PCBs.     

Location and sequence analysis of a 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoate hydrolase-encoding gene (bpdF) of the biphenyl/ polychlorinated biphenyl degradation pathway in Rhodococcus sp. M5

The 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoate (HOPD) hydrolase-encoding gene (bpdF) in the biphenyl (BP)/polychlorinated biphenyl (PCB)-degrading bacterium, Rhodococcus sp. M5 (M5), was found to be located within a 4.5-kb HindIII-BamHI genomic DNA that was 5.4 kb downstream from the bpdC1C2BADE gene cluster. The deduced amino acid (aa) sequence of bpdF revealed that the hydrolase contains 297 aa (32679 Da) that was verified by expression in the Escherichia coli T7 RNA polymerase/promoter system. Unlike previously known HOPD hydrolases, the aa sequence of BpdF appears unique. Interestingly, all HOPD hydrolases and related proteins from the phenol and toluene/ xylene degradation pathways, were found to have a bias in the codon usage in the catalytic Ser within the conserved VGNS(M/F)GG motif.     

Crystallization of the terminal oxygenase component of biphenyl dioxygenase derived from Rhodococcus sp. strain RHA1

The terminal oxygenase component of the biphenyl dioxygenase (BphA1A2 complex) was over-expressed with a novel over expression system in recombinant Rhodococcus strain and purified. The purified enzyme has been crystallized by the hanging drop vapor diffusion method and subjected to X-ray diffraction analysis. The crystals belong to the tetragonal system in the space group P4(1)2(1)2 or P4(3)2(1)2 and diffract to better than 2.2A resolution.     

Discovery of a eugenol oxidase from Rhodococcus sp. strain RHA1

A gene encoding a eugenol oxidase was identified in the genome from Rhodococcus sp. strain RHA1. The bacterial FAD-containing oxidase shares 45% amino acid sequence identity with vanillyl alcohol oxidase from the fungus Penicillium simplicissimum. Eugenol oxidase could be expressed at high levels in Escherichia coli, which allowed purification of 160 mg of eugenol oxidase from 1 L of culture. Gel permeation experiments and macromolecular MS revealed that the enzyme forms homodimers. Eugenol oxidase is partly expressed in the apo form, but can be fully flavinylated by the addition of FAD. Cofactor incorporation involves the formation of a covalent protein-FAD linkage, which is formed autocatalytically. Modeling using the vanillyl alcohol oxidase structure indicates that the FAD cofactor is tethered to His390 in eugenol oxidase. The model also provides a structural explanation for the observation that eugenol oxidase is dimeric whereas vanillyl alcohol oxidase is octameric. The bacterial oxidase efficiently oxidizes eugenol into coniferyl alcohol (KM=1.0 microM, kcat=3.1 s-1). Vanillyl alcohol and 5-indanol are also readily accepted as substrates, whereas other phenolic compounds (vanillylamine, 4-ethylguaiacol) are converted with relatively poor catalytic efficiencies. The catalytic efficiencies with the identified substrates are strikingly different when compared with vanillyl alcohol oxidase. The ability to efficiently convert eugenol may facilitate biotechnological valorization of this natural aromatic compound.     

Crystal structure of the terminal oxygenase component of biphenyl dioxygenase derived from Rhodococcus sp. strain RHA1

Biphenyl dioxygenase is the enzyme that catalyzes the stereospecific dioxygenation of the aromatic ring. This enzyme has attracted the attention of researchers due to its ability to oxidize polychlorinated biphenyls, which is one of the serious environmental contaminants. We determined the crystal structure of the terminal oxygenase component of the biphenyl dioxygenase (BphA1A2) derived from Rhodococcus strain sp. RHA1 in substrate-free and complex forms. These crystal structures revealed that the substrate-binding pocket makes significant conformational changes upon substrate binding to accommodate the substrate into the pocket. Our analysis of the crystal structures suggested that the residues in the substrate-binding pocket can be classified into three groups, which, respectively, seem to be responsible for the catalytic reaction, the orientation/conformation of the substrate, and the conformational changes of the substrate-binding pocket. The cooperative actions of residues in the three groups seem to determine the substrate specificity of the enzyme.     

Functional characterization of a catabolic plasmid from polychlorinated- biphenyl-degrading Rhodococcus sp. strain RHA1

Rhodococcus sp. strain RHA1, a potent polychlorinated-biphenyl (PCB)-degrading strain, contains three linear plasmids ranging in size from 330 to 1,100 kb. As part of a genome sequencing project, we report here the complete sequence and characterization of the smallest and least-well-characterized of the RHA1 plasmids, pRHL3. The plasmid is an actinomycete invertron, containing large terminal inverted repeats with a tightly associated protein and a predicted open reading frame (ORF) that is similar to that of a mycobacterial rep gene. The pRHL3 plasmid has 300 putative genes, almost 21% of which are predicted to have a catabolic function. Most of these are organized into three clusters. One of the catabolic clusters was predicted to include limonene degradation genes. Consistent with this prediction, RHA1 grew on limonene, carveol, or carvone as the sole carbon source. The plasmid carries three cytochrome P450-encoding (CYP) genes, a finding consistent with the high number of CYP genes found in other actinomycetes. Two of the CYP genes appear to belong to novel families; the third belongs to CYP family 116 but appears to belong to a novel class based on the predicted domain structure of its reductase. Analyses indicate that pRHL3 also contains four putative "genomic islands" (likely to have been acquired by horizontal transfer), insertion sequence elements, 19 transposase genes, and a duplication that spans two ORFs. One of the genomic islands appears to encode resistance to heavy metals. The plasmid does not appear to contain any housekeeping genes. However, each of the three catabolic clusters contains related genes that appear to be involved in glucose metabolism.