Biotransformation of 1,2,3-Tri- and 1,2,3,4,7,8-Hexachlorodibenzo-p- Dioxin by Sphingomonas wittichii Strain RW1

Sphingomonas wittichii RW1 is able to catabolize 1,2,3,4-tetrachlorodibenzo-p-dioxin (H. B. Hong, Y. S. Chang, I. H. Nam, P. Fortnagel, and S. Schmidt, Appl. Environ. Microbiol. 68:2584-2588, 2002). Here we demonstrate the aerobic bacterial catabolism of the ubiquitous toxic diaryl ether pollutant 1,2,3,4,7,8-hexachlorodibenzo-p-dioxin by this strain. The products of this biotransformation were identified as tetrachlorocatechol and 2-methoxy-3,4,5,6-tetrachlorophenol by comparing mass spectra recorded before and after n-butylboronate and N,O-bis(trimethylsilyl)-trifluoroacetamide derivatization with those of authentic compounds. Additional experiments showed that the less-chlorinated 1,2,3,7,8-pentachlorodibenzo-p-dioxin was not transformed by the strain RW1. The importance of substitution patterns for the degradability of individual congeners was illustrated by the fact that the 1,2,3-trichlorodibenzo-p-dioxin was catabolized to yield 3,4,5-trichlorocatechol, whereas the 2,3,7-trichlorodibenzo-p-dioxin was not attacked.     

Isolation and characterization of dibenzofuran-degrading bacteria

Two bacterial strains capable of utilizing dibenzofuran (DF) as a sole carbon source were isolated from soil samples of reclaimed land. The strains designated HL1 and HL7 were identified as Klebsiella sp. and Sphingomonas sp., respectively, on the basis of biochemical characteristics and the sequences of the 16S ribosomal DNA. Sphingomonas sp. strain HL7 degraded non-, mono- and also dichlorinated DF and dibenzo-p-dioxin (DD). Klebsiella sp. strain HL1 was able to degrade non- and monochlorinated DFs and DDs, but not dichlorinated ones. The metabolites formed from DF by strains HL1 and HL7 were similar to those by dioxin-degrading bacteria Sphingomonas sp. strain RW1 except for salicylic acid and catechol. Strain HL7 had a gene homologous to that encoding the dioxin dioxygenase alpha-subunit (dxnA1) gene of Sphingomonas sp. strain RW1. However, Southern hybridization analysis showed that the size of an EcoRV-digested genomic fragment involving the dioxin dioxygenase gene of strain HL7 was smaller than that of strain RW1, and that strain HL1 did not have the homologous gene. Strains HL1 and HL7 provided useful information regarding the dioxygenase genes.     

A second [2Fe-2S] ferredoxin from Sphingomonas sp. Strain RW1 can function as an electron donor for the dioxin dioxygenase

The first step in the degradation of dibenzofuran and dibenzo-p-dioxin by Sphingomonas sp. strain RW1 is carried out by dioxin dioxygenase (DxnA1A2), a ring-dihydroxylating enzyme. An open reading frame (fdx3) that could potentially specify a new ferredoxin has been identified downstream of dxnA1A2, a two-cistron gene (J. Armengaud, B. Happe, and K. N. Timmis, J. Bacteriol. 180:3954-3966, 1998). In the present study, we report a biochemical analysis of Fdx3 produced in Escherichia coli. This third ferredoxin thus far identified in Sphingomonas sp. strain RW1 contained a putidaredoxin-type [2Fe-2S] cluster which was characterized by UV-visible absorption spectrophotometry and electron paramagnetic resonance spectroscopy. The midpoint redox potential of this ferredoxin (E'(0) = -247 +/- 10 mV versus normal hydrogen electrode at pH 8.0) is similar to that exhibited by Fdx1 (-245 mV), a homologous ferredoxin previously characterized in Sphingomonas sp. strain RW1. In in vitro assays, Fdx3 can be reduced by RedA2 (a reductase similar to class I cytochrome P-450 reductases), previously isolated from Sphingomonas sp. strain RW1. RedA2 exhibits a K(m) value of 3.2 +/- 0.3 microM for Fdx3. In vivo coexpression of fdx3 and redA2 with dxnA1A2 confirmed that Fdx3 can serve as an electron donor for the dioxin dioxygenase.     

Isolation and characterization of the genes encoding a novel oxygenase component of angular dioxygenase from the gram-positive dibenzofuran-degrader Terrabacter sp. strain DBF63

A gram-positive bacterium Terrabacter sp. strain DBF63 is able to degrade dibenzofuran (DF) via initial dioxygenation by a novel angular dioxygenase. The dbfA1 and dbfA2 genes, which encode the large and small subunits of the dibenzofuran 4,4a-dioxygenase (DFDO), respectively, were isolated by a polymerase chain reaction-based method. DbfA1 and DbfA2 showed moderate homology to the large and small subunits of other ring-hydroxylating dioxygenases (less than 40%), respectively, and some motifs such as the Fe(II) binding site and the [2Fe-2S] cluster ligands were conserved in DbfA1. DFDO activity was confirmed in Escherichia coli cells containing the cloned dbfA1 and dbfA2 genes with the complementation of nonspecific ferredoxin and ferredoxin reductase component of E. coli. Under this condition, these cells exhibited angular dioxygenation of DF and dibenzo-p-dioxin, and monooxygenation of fluorene, but not angular dioxygenation of carbazole, xanthene, and phenoxathiin. Phylogenetic analysis revealed that DbfA1 formed a branch with recently reported large subunits of polycyclic aromatic hydrocarbon (PAH) dioxygenase from gram-positive bacteria but did not cluster with that of other angular dioxygenases, i.e., DxnA1 from Sphingomonas sp. strain RW1 [Armengaud, J., Happe, B., and Timmis, K. N. J. Bacteriol. 180, 3954-3966, 1998] and CarAa from Pseudomonas sp. strain CA10 [Sato, S., Nam, J.-W., Kasuga, K., Nojiri, H., Yamane, H., and Omori, T. J. Bacteriol. 179, 4850-4858, 1997].     

Use of monoclonal antibodies against dibenzo-p-dioxin degrading Sphingomonas sp. strain RW1

I.S. THAKUR. 1996. A monoclonal antibody prepared against surface antigen of Sphingomonas sp. strain RW1 was used for the direct detection of RW1-like organisms in environmental samples by epifluorescence microscopy and subsequent confirmation by Western blot. Of the 76 samples collected from various sources and probed using epifluorescence, only one sample, effluent from paper and pulp processing, gave a positive result. The effluent was cultured and yielded an organism which, by Western blot analysis, was shown to contain the 28 kDa protein recognized by the monoclonal antibody.     

Degradation of Chlorinated Dibenzofurans and Dibenzo-p-Dioxins by Sphingomonas sp. Strain RW1

The ability of the dibenzofuran- and dibenzo-p-dioxin-mineralizing bacterium Sphingomonas sp. strain RW1 (R.-M. Wittich, H. Wilkes, V. Sinnwell, W. Francke, and P. Fortnagel, Appl. Environ. Microbiol. 58:1005-1010, 1992) to oxidize chlorinated derivatives of dibenzofuran and dibenzo-p-dioxin was analyzed. Strain RW1 degraded several mono- and dichlorinated dibenzofurans and dibenzo-p-dioxins, but it did not degrade more highly chlorinated congeners. Most mono- and dichlorinated dibenzofurans and dibenzo-p-dioxins investigated in this study were degraded to the corresponding mono- and dichlorinated salicylates and catechols, respectively, together with salicylate and catechol. This indicates an initial dioxygenolytic attack on the substituted as well as on the nonsubstituted aromatic nucleus of most of the target compounds. Strain RW1 could not grow at the expense of monochlorinated dibenzo-p-dioxins and dibenzofurans as carbon sources, with the exception of 4-chlorodibenzofuran, which was stoichiometrically converted to 3-chlorosalicylate.     

Mineralization of 4-Chlorodibenzofuran by a Consortium Consisting of Sphingomonas sp. Strain RW1 and Burkholderia sp. Strain JWS

The dibenzofuran-degrading bacterium Sphingomonas sp. strain RW1 (R.-M. Wittich, H. Wilkes, V. Sinnwell, W. Francke, and P. Fortnagel, Appl. Environ. Microbiol. 58:1005-1010, 1992) attacks 4-chlorodibenzofuran on the unsubstituted aromatic ring via distal dioxygenation adjacent to the ether bridge to produce 3(prm1)-chloro-2,2(prm1),3-trihydroxybiphenyl, which was identified by nuclear magnetic resonance spectroscopy and mass spectrometry. The compound is subsequently meta cleaved, and the respective intermediate is hydrolyzed to form a C-5 moiety, which is further degraded to Krebs cycle intermediates and to 3-chlorosalicylate. This dead-end product is released into the culture medium. A coculture of strain RW1 and the 3,5-dichlorosalicylate-degrading strain Burkholderia sp. strain JWS (A. Schindowski, R.-M. Wittich, and P. Fortnagel, FEMS Microbiol. Lett. 84:63-70, 1991) is able to completely degrade 4-chlorodibenzofuran with concomitant release of Cl(sup-) and formation of biomass.     

Carboxymethylcellulase produced by facultative bacteria from the hind-gut of the termite Reticulitermes hesperus.

Bacillus cereus RW1 and Serratia marcescens RW3, isolated from the hind-gut of the termite Reticulitermes hesperus, both grew well on mesquite wood and produced moderate amounts of carboxymethylcellulase. Carboxymethylcellulose (CMC) gels were depolymerized rapidly by B. cereus RW1 and slowly by S. marcescens RW3. The depolymerization of CMC was pH and temperature sensitive. Depolymerization of gels by growing cultures of B. cereus RW1 and the action of cell-free extracts of B. cereus RW1 on CMC sols were optimum at pH 6.0 and 5.5, respectively. Glucose and cellobiose increased the rate of CMC gel depolymerization. Enzyme synthesis rather than growth was stimulated by the addition of glucose to a culture of RW1 growing on a non-cellulosic substrate. Bacillus cereus RW1 produced both cell-free and cell-bound carboxymethylcellulase.     

Biotransformation of 2,7-dichloro- and 1,2,3,4-tetrachlorodibenzo-p-dioxin by Sphingomonas wittichii RW1

Aerobic biotransformation of the diaryl ethers 2,7-dichlorodibenzo-p-dioxin and 1,2,3,4-tetrachlorodibenzo-p-dioxin by the dibenzo-p-dioxin-utilizing strain Sphingomonas wittichii RW1, producing corresponding metabolites, was demonstrated for the first time. Our strain transformed 2,7-dichlorodibenzo-p-dioxin, yielding 4-chlorocatechol, and 1,2,3,4-tetrachlorodibenzo-p-dioxin, producing 3,4,5,6-tetrachlorocatechol and 2-methoxy-3,4,5,6-tetrachlorophenol; all of these compounds were unequivocally identified by mass spectrometry both before and after N,O-bis(trimethylsilyl)-trifluoroacetamide derivatization by comparison with authentic standards. Additional experiments showed that strain RW1 formed a second metabolite, 2-methoxy-3,4,5,6-tetrachlorophenol, from the original degradation product, 3,4,5,6-tetrachlorocatechol, by methylation of one of the two hydroxy substituents.     

Degradation of hydrogen sulfide by Xanthomonas sp. strain DY44 isolated from peat.

Xanthomonas sp. strain DY44, capable of degrading H2S, was isolated from dimethyl disulfide-acclimated peat. This bacterium removed H2S either as a single gas or in the presence of the sulfur-containing compounds methanethiol, dimethyl sulfide, and dimethyl disulfide. The maximum specific H2S removal rate, obtained in the late stationary phase, was 3.92 mmol g of dry cells-1 h-1 (6.7 x 10(-16) mol cell-1 h-1) at pH 7 and 30 degrees C through a batch experiment in a basal mineral medium. Since Xanthomonas sp. strain DY44 exhibited no autotrophic growth with H2S, the H2S removal was judged not to be a consequence of chemolithotrophic activity. By using X-ray photoelectron spectroscopy, the metabolic product of H2S oxidation was determined to be polysulfide, which has properties very similar to those of elemental sulfur. Autoclaved cells (120 degrees C, 20 min) did not show H2S degradation, but cells killed by gamma-irradiation and cell extracts both oxidized H2S, suggesting the existence of a heat-labile intracellular enzymatic system for H2S oxidation. When Xanthomonas sp. strain DY44 was inoculated into fibrous peat, this strain degraded H2S without lag time, suggesting that it will be a good candidate for maintaining high H2S removability in the treatment of exhaust gases.     

Degradation of hydrogen sulfide by Xanthomonas sp. strain DY44 isolated from peat.

Xanthomonas sp. strain DY44, capable of degrading H2S, was isolated from dimethyl disulfide-acclimated peat. This bacterium removed H2S either as a single gas or in the presence of the sulfur-containing compounds methanethiol, dimethyl sulfide, and dimethyl disulfide. The maximum specific H2S removal rate, obtained in the late stationary phase, was 3.92 mmol g of dry cells-1 h-1 (6.7 x 10(-16) mol cell-1 h-1) at pH 7 and 30 degrees C through a batch experiment in a basal mineral medium. Since Xanthomonas sp. strain DY44 exhibited no autotrophic growth with H2S, the H2S removal was judged not to be a consequence of chemolithotrophic activity. By using X-ray photoelectron spectroscopy, the metabolic product of H2S oxidation was determined to be polysulfide, which has properties very similar to those of elemental sulfur. Autoclaved cells (120 degrees C, 20 min) did not show H2S degradation, but cells killed by gamma-irradiation and cell extracts both oxidized H2S, suggesting the existence of a heat-labile intracellular enzymatic system for H2S oxidation. When Xanthomonas sp. strain DY44 was inoculated into fibrous peat, this strain degraded H2S without lag time, suggesting that it will be a good candidate for maintaining high H2S removability in the treatment of exhaust gases.     

Identification and phenotypic characterization of Sphingomonas wittichii strain RW1 by peptide mass fingerprinting using matrix-assisted laser desorption ionization-time of flight mass spectrometry

Mass spectrometry is a potentially attractive means of monitoring the survival and efficacy of bioaugmentation agents, such as the dioxin-mineralizing bacterium Sphingomonas wittichii strain RW1. The biotransformation activity of RW1 phenotypes is determined primarily by the presence and concentration of the dioxin dioxygenase, an enzyme initiating the degradation of both dibenzo-p-dioxin and dibenzofuran (DF). We explored the possibility of identifying and characterizing putative cultures of RW1 by peptide mass fingerprinting (PMF) targeting this characteristic phenotypic biomarker. The proteome from cells of RW1--grown on various media in the presence and absence of DF--was partially purified, tryptically digested, and analyzed using matrix-assisted laser desorption ionization-time of flight mass spectrometry. Mascot online database queries allowed statistically significant identification of RW1 in disrupted, digested cells (P < 0.01 to 0.05) and in digested whole-cell extracts (P < 0.00001 to 0.05) containing hundreds of proteins, as determined by two-dimensional gel electrophoresis. Up to 14 peptide ions of the alpha subunit of the dioxin dioxygenase (43% protein coverage) were detected in individual samples. A minimum of 10(7) DF-grown cells was required to identify dioxin degradation-enabled phenotypes. The technique hinges on the detection of multiple characteristic peptides of a biomarker that can reveal at once the identity and phenotypic properties of the microbial host expressing the protein. The results demonstrate the power of PMF of minimally processed microbial cultures as a sensitive and specific technique for the positive identification and phenotypic characterization of certain microorganisms used in biotechnology and bioremediation.     

Superior survival and degradation of dibenzo-p-dioxin and dibenzofuran in soil by soil-adapted Sphingomonas sp. strain RW1

The dibenzo-p-dioxin(DD)- and dibenzofuran(DF)-degrading bacterium, Sphingomonas sp. strain RW1, was tagged by insertion of a mini-Tn5 lacZ transposon in order to follow its fate in complex laboratory soil systems. The tagged strain was tested for its ability to survive in soil and degrade DF and DD applied at a concentration of 1 mg/g. Bacteria pre-adapted to soil conditions were found to survive better in DF- and DD-amended soil and degrade the substrate more efficiently than bacteria that had not been subjected to pre-adaptation. The concentration of soil-applied DF and DD, individually and in combination, decreased to less than 2% of the original concentrations within 3 weeks of addition of the RW1 derivative, accompanied by a short, but significant exponential increase in RW1 viable cells. During the same period the native bacterial population in soil was stable while viable fungi declined. Received: 12 November 1996 / Received revision: 21 February 1997 / Accepted: 22 February 1997     

Removal of dibenzofuran, dibenzo-p-dioxin, and 2-chlorodibenzo-p-dioxin from soils inoculated with Sphingomonas sp. strain RW1

Removal of dibenzofuran, dibenzo-p-dioxin, and 2-chlorodibenzo-p-dioxin (2-CDD) (10 ppm each) from soil microcosms to final concentrations in the parts-per-billion range was affected by the addition of Sphingomonas sp. strain RW1. Rates and extents of removal were influenced by the density of RW1 organisms. For 2-CDD, the rate of removal was dependent on the content of soil organic matter (SOM), with half-life values ranging from 5.8 h (0% SOM) to 26.3 h (5.5% SOM).     

Identification and Phenotypic Characterization of Sphingomonas wittichii Strain RW1 by Peptide Mass Fingerprinting Using Matrix-Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry

Mass spectrometry is a potentially attractive means of monitoring the survival and efficacy of bioaugmentation agents, such as the dioxin-mineralizing bacterium Sphingomonas wittichii strain RW1. The biotransformation activity of RW1 phenotypes is determined primarily by the presence and concentration of the dioxin dioxygenase, an enzyme initiating the degradation of both dibenzo-p-dioxin and dibenzofuran (DF). We explored the possibility of identifying and characterizing putative cultures of RW1 by peptide mass fingerprinting (PMF) targeting this characteristic phenotypic biomarker. The proteome from cells of RW1—grown on various media in the presence and absence of DF—was partially purified, tryptically digested, and analyzed using matrix-assisted laser desorption ionization-time of flight mass spectrometry. Mascot online database queries allowed statistically significant identification of RW1 in disrupted, digested cells (P < 0.01 to 0.05) and in digested whole-cell extracts (P < 0.00001 to 0.05) containing hundreds of proteins, as determined by two-dimensional gel electrophoresis. Up to 14 peptide ions of the alpha subunit of the dioxin dioxygenase (43% protein coverage) were detected in individual samples. A minimum of 10⁷ DF-grown cells was required to identify dioxin degradation-enabled phenotypes. The technique hinges on the detection of multiple characteristic peptides of a biomarker that can reveal at once the identity and phenotypic properties of the microbial host expressing the protein. The results demonstrate the power of PMF of minimally processed microbial cultures as a sensitive and specific technique for the positive identification and phenotypic characterization of certain microorganisms used in biotechnology and bioremediation.     

Dynamic stability and compensatory stepping responses during anterior gait–slip perturbations in people with chronic hemiparetic stroke

To examine the control of dynamic stability and characteristics of the compensatory stepping responses to an unexpected anterior gait slip induced under the non-involved limb in people with hemi-paretic stroke (PwHS) and to examine any resulting adaptive changes in these on the second slip due to experience from prior slip exposure. Ten PwHS experienced overground slip (S1) during walking on the laboratory walkway after 5–8 regular walking (RW) trials followed by a second consecutive slip trial (S2). The slip outcome (backward loss of balance, BLOB and no loss of balance, NLOB) and COM state (i.e. its COM position and velocity) stability were examined between the RW and S1 and S1 and S2 at touchdown (TD) of non-involved limb and at liftoff (LO) of the contralateral limb. At TD there was no difference in stability between RW and S1, however at LO, subjects demonstrated a lower stability on S1 than RW resulting in a 100% backward loss of balance (BLOB) with compensatory stepping response (recovery step, RS, 4/10 or aborted step, AS, 6/10). On S2, although there was no change in stability at TD, there was a significant improvement in stability at LO with a 40% decrease in BLOB. There was also a change in step strategy with a decrease in AS response (60% to 35%, p<0.05) which was replaced by an increase in the ability to step (increased compensatory step length, p<0.05) either via a recovery step or a walkover step. PwHS have the ability to reactively control COM state stability to decrease fall-risk upon a novel slip; prior exposure to a slip did not significantly alter feedforward control but improved the ability to use such feedback control for improved slip outcomes.     

Stability of mutations in a Sphingomonas strain.

Sphingomonas sp. strain RW1 is able to mineralise dibenzofuran and dibenzo-p-dioxin. Three mutants were constructed that could not use dibenzofuran or dibenzo-p-dioxin as a carbon source but were able to grow with the succeeding metabolites of the pathway. Two different mutagenic agents were applied, a chemical treatment with 1-methyl-3-nitro-1-nitrosoguanidine, resulting in mutants RW1-N6 and RW1-N7, and a biological insertion mutagenesis with the mini-Tn5 transposon pBSL118, resulting in mutant RW1-M3. Southern blot analysis and PCR experiments confirmed a single insertion of the mini-Tn5 into one of the genes coding for the oxygenase component of the dibenzofuran 4,4a-dioxygenase system. The genetic stability of these mutants was examined after growth with complex medium under nonselective conditions. All three mutants failed to revert to wild-type metabolic functions.     

Hydrogenophaga intermedia sp. nov., a 4-aminobenzenesulfonate degrading organism

The taxonomic status of a gram-negative, oxidase positive rod (strain S1) able to degrade 4-aminobenzenesulfonate was studied using a polyphasic approach. Chemotaxonomic investigations of quinones and polar lipids established the allocation of this strain to the beta-subclass of the Proteobacteria and revealed similarities to Hydrogenophaga palleronii. 16S rRNA sequence comparisons demonstrated that this strain clusters phylogenetically with H. palleronii and H. taeniospiralis, but clearly represents a new species. The fatty acid patterns and substrate utilization profile displayed similarity to the characteristics of the four validly published species of Hydrogenophaga, although clear differentiating characters were also observed. No close similarities between the type strains of H. palleronii and H. taeniospiralis were detected in hybridization experiments with the genomic DNAs. On basis of these results, the new species Hydrogenophaga intermedia sp. nov. is proposed, with the type strain S1T (= DSM 5680).     

Enrichment of a dioxin-dehalogenating Dehalococcoides species in two-liquid phase cultures

Enrichment cultures capable of reductively dechlorinating 1,2,4-trichlorodibenzo-p-dioxin (1,2,4-TrCDD) were shown to dechlorinate 1,2,3-trichlorobenzene (1,2,3-TrCB) to 1,3-dichlorobenzene. To test if this activity can be used to enrich for dioxin-dechlorinating bacteria, a two-liquid phase cultivation with 200 mM 1,2,3-TrCB dissolved in hexadecane was established. During the dechlorination of 1,2,3-TrCB, the number of 1,2,4-TrCDD-dechlorinating bacteria increased by four orders of magnitude, eventually accounting for 11% of the total cell number. Characterization of the bacterial communities of the initial dioxin-dechlorinating culture and of the trichlorobenzene enrichments by restriction fragment length polymorphism (RFLP) analysis of cloned 16S rRNA genes revealed a proportional increase of nine different sequence types, one representing a Dehalococcoides strain. Inhibition of methanogens further enhanced the rate of chlorobenzene dehalogenation and also resulted in a rapid dechlorination of 1,2,3,4-tetrachlorodibenzo-p-dioxin that was applied via a hexadecane phase. The further enrichment was monitored by terminal RFLP, quantitative real-time PCR and microscopy, and aimed at the reduction of the accompanying non-dehalogenating populations by using different combinations of electron donors and the application of antibiotics. Hydrogen as the sole electron donor proved to be less efficient due to the co-enrichment of acetogens. The novel Dehalococcoides strain DCMB5 was enriched up to 50% by the cultivation with organic acids, hydrogen and vancomycin, and was finally purified by conventional isolation techniques.     

Detection and characterization of conjugative degradative plasmids in xenobiotic-degrading Sphingomonas strains

A systematic survey for the presence of plasmids in 17 different xenobiotic-degrading Sphingomonas strains was performed. In almost all analyzed strains, two to five plasmids with sizes of about 50 to 500 kb were detected by using pulsed-field gel electrophoresis. A comparison of plasmid preparations untreated or treated with S1 nuclease suggested that, in general, Sphingomonas plasmids are circular. Hybridization experiments with labeled gene probes suggested that large plasmids are involved in the degradation of dibenzo-p-dioxin, dibenzofuran, and naphthalenesulfonates in S. wittichii RW1, Sphingomonas sp. HH69, and S. xenophaga BN6, respectively. The plasmids which are responsible for the degradation of naphthalene, biphenyl, and toluene by S. aromaticivorans F199 (pNL1) and of naphthalenesulfonates by S. xenophaga BN6 (pBN6) were site-specifically labeled with a kanamycin resistance cassette. The conjugative transfer of these labeled plasmids was attempted with various bacterial strains as putative recipient strains. Thus, a conjugative transfer of plasmid pBN6 from S. xenophaga BN6 to a cured mutant of strain BN6 and to Sphingomonas sp. SS3 was observed. The conjugation experiments with plasmid pNL1 suggested a broader host range of this plasmid, because it was transferred without any obvious structural changes to S. yanoikuyae B1, Sphingomonas sp. SS3, and S. herbicidovorans. In contrast, major plasmid rearrangements were observed in the transconjugants after the transfer of plasmid pNL1 to Sphingomonas sp. HH69 and of pBN6 to Sphingomonas sp. SS3. No indications for the transfer of a Sphingomonas plasmid to bacteria outside of the Sphingomonadaceae were obtained.