Fate of free DNA and transformation of the oral bacterium Streptococcus gordonii DL1 by plasmid DNA in human saliva

Competitive PCR was used to monitor the survival of a 520-bp DNA target sequence from a recombinant plasmid, pVACMC1, after admixture of the plasmid with freshly sampled human saliva. The fraction of the target remaining amplifiable ranged from 40 to 65% after 10 min of exposure to saliva samples from five subjects and from 6 to 25% after 60 min of exposure. pVACMC1 plasmid DNA that had been exposed to degradation by fresh saliva was capable of transforming naturally competent Streptococcus gordonii DL1 to erythromycin resistance, although transforming activity decreased rapidly, with a half-life of approximately 50 s. S. gordonii DL1 transformants were obtained in the presence of filter-sterilized saliva and a 1-microg/ml final concentration of pVACMC1 DNA. Addition of filter-sterilized saliva instead of heat-inactivated horse serum to S. gordonii DL1 cells induced competence, although with slightly lower efficiency. These findings indicate that DNA released from bacteria or food sources within the mouth has the potential to transform naturally competent oral bacteria. However, further investigations are needed to establish whether transformation of oral bacteria can occur at significant frequencies in vivo.

Estimation of the relative abundance of different Bacteroides and Prevotella ribotypes in gut samples by restriction enzyme profiling of PCR-amplified 16S rRNA gene sequences

We describe an approach for determining the genetic composition of Bacteroides and Prevotella populations in gut contents based on selective amplification of 16S rRNA gene sequences (rDNA) followed by cleavage of the amplified material with restriction enzymes. The relative contributions of different ribotypes to total Bacteroides and Prevotella 16S rDNA are estimated after end labelling of one of the PCR primers, and the contribution of Bacteroides and Prevotella sequences to total eubacterial 16S rDNA is estimated by measuring the binding of oligonucleotide probes to amplified DNA. Bacteroides and Prevotella 16S rDNA accounted for between 12 and 62% of total eubacterial 16S rDNA in samples of ruminal contents from six sheep and a cow. Ribotypes 4, 5, 6, and 7, which include most cultivated rumen Prevotella strains, together accounted for between 20 and 86% of the total amplified Bacteroides and Prevotella rDNA in these samples. The most abundant Bacteroides or Prevotella ribotype in four animals, however, was ribotype 8, for which there is only one known cultured isolate, while ribotypes 1 and 2, which include many colonic Bacteroides spp., were the most abundant in two animals. This indicates that some abundant Bacteroides and Prevotella groups in the rumen are underrepresented among cultured rumen Prevotella isolates. The approach described here provides a rapid, convenient, and widely applicable method for comparing the genotypic composition of bacterial populations in gut samples.

A cysteine desulphurase gene from the cellulolytic rumen anaerobe Ruminococcus flavefaciens

A gene whose predicted product shows 40-50% sequence identity with the products of nifS genes from nitrogen-fixing bacteria was found downstream from a cellulase gene in a DNA fragment from the cellulolytic rumen anaerobe, Ruminococcus flavefaciens 17. The R. flavefaciens gene product released sulphur from l-cysteine when expressed in Escherichia coli, indicating that the R. flavefaciens NifS enzyme may play a role in sulphuration, perhaps, as in nitrogen-fixing bacteria, supplying sulphur to FeS proteins. Sequences hybridising with the R. flavefaciens 17 nifS-like gene were also detected in R. flavefaciens 007 and in R. albus SY3.

Inhibitory activity of gut bacteria against Escherichia coli O157 mediated by dietary plant metabolites

Under both aerobic and anaerobic conditions, the growth of Escherichia coli O157 strain NCTC 12,900 was inhibited by the coumarins esculetin, umbelliferone and scopoletin, but not by the coumarin glycoside esculin. Esculin-hydrolysing bacteria from the rumen, the pig gut and the human gut inhibited growth of E. coli in an overlay-plate assay in the presence of esculin. The combined effect of esculetin and volatile fatty acids was greater than the effect of either factor alone suggesting that coumarin glycosides in the diet might reduce the growth or survival of E. coli O157 in the gut. Adding esculin to incubations of mixed rumen contents significantly reduced the survival of E. coli O157.

Large plasmids in ruminal strains of Selenomonas ruminantium

The plasmid content of six different isolates of Selenomonas ruminantium from the rumen of sheep, cows or goats was examined by electron microscopy. In addition to small plasmids (< 12 kb) studied previously, all six strains contained at least one plasmid larger than 20 kb. Plasmid sizes of 1.4, 2.1, 2.4, 5.0, 6.2, 20.4, 20.8, 22.7, 23.3, 29.3, 30.7, 34.4 and 42.6 kb were estimated from contour length measurements. DNA-DNA hybridization experiments revealed homology among the large plasmids from five strains, while the 20.8 kb plasmid from a sixth isolate showed no apparent relationship with the plasmids of the other strains.

The rumen microbial ecosystem--some recent developments

The rumen of sheep and cattle represents a mobile, self-sustaining fermentation system for plant material. Analysis of the rumen flora continues to provide fundamental knowledge of anaerobic microbiology and is now yielding microbial genes that have potential in biotechnology. Recent research has provided fascinating glimpses into the microbial enzyme systems that degrade plant material and into the complex interplay among members of the rumen community.

Isolation and overexpression of a gene encoding an extracellular beta-(1,3-1,4)-glucanase from Streptococcus bovis JB1

Streptococcus bovis JB1 was found to produce a 25-kDa extracellular enzyme active against beta-(1,3-1,4)-glucans. A gene was isolated encoding a specific beta-(1,3-1,4)-glucanase that corresponds to this size and belongs to glycoside hydrolase family 16. A 4- to 10-fold increase in supernatant beta-glucanase activity was obtained when the cloned beta-glucanase gene was reintroduced into S. bovis JB1 by use of constructs based on the plasmid vector pTRW10 or pIL253. The beta-(1,3-1,4)-glucanase gene was also expressed upon introduction of the pTRW10 construct pTRWL1R into Lactococcus lactis IL2661 and Enterococcus faecalis JH2-SS, although extracellular activity was 8- to 50-fold lower than that in S. bovis JB1. The beta-(1,3-1,4)-glucanase purified from the culture supernatant of S. bovis JB1 carrying pTRWL1R showed a K(m) of 2.8 mg per ml and a Vmax of 338 mumol of glucose equivalents per min per mg of protein with barley beta-glucan as the substrate. The S. bovis beta-(1,3-1,4)-glucanase may contribute to the ability of this bacterium to utilize starch by degrading structural polysaccharides present in endosperm cell walls.

High-frequency transfer of a naturally occurring chromosomal tetracycline resistance element in the ruminal anaerobe Butyrivibrio fibrisolvens

Butyrivibrio fibrisolvens strains resistant to tetracycline were isolated from the bovine rumen. Two of three Tcr B. fibrisolvens tested were able to donate tetracycline resistance at frequencies ranging from 10(-7) to 10(-1) per donor cell in anaerobic filter matings to a rifampin-resistant mutant of the type strain of B.fibrisolvens, 2221R. The recipient strain 2221R exhibited rapid autoaggregation, which might be a factor in the high transfer rates observed. Tcr transconjugants of B. fibrisolvens 2221R were also capable of further transferring tetracycline resistance to a fusidic acid-resistant mutant, 2221F. Comparison of genomic DNAs by pulsed-field gel electrophoresis demonstrated altered band profiles in transconjugants, consistent with the acquisition of a large mobile chromosomal element. The transferable elements from the two B. fibrisolvens donors 1.23 and 1.230 (TnB123 and TnB1230, respectively) showed the same preferred insertion site in the B. fibrisolvens 2221R chromosome and are likely to be similar, or identical, elements. Hybridization experiments showed no close relationship between TnB1230 and int-xis regions from Tn916 or Tn5253. Although DNA from the B. fibrisolvens donor strains hybridized with probes carrying tet(M) or tet(O) sequences, transconjugants were found to have acquired a distinct band that hybridized only weakly with these probes, suggesting that a second, distantly related Tcr determinant had been transferred.

Dockerin-like sequences in cellulases and xylanases from the rumen cellulolytic bacterium Ruminococcus flavefaciens

Recent analysis of the endA cellulase gene from Ruminococcus flavefaciens 17 has revealed that it encodes a product of 759 amino acids that provides the first example of a multidomain cellulase from a Ruminococcus sp. Following the family 5 catalytic domain in the predicted EndA enzyme is a 282 amino acid domain of unknown function for which no close relationship was found to other protein sequences. However, the C-terminal sequences of EndA contain a 34 amino acid threonine-rich linker connected to an 81 amino acid region, both of which show strong similarities to sequences present in two xylanases from R. flavefaciens 17. A distant relationship is evident between regions of the 80 amino acid sequences of EndA, XynD and XynB and the duplicated 23 amino acid dockerin sequences found in cellulolytic Clostridium sp., suggesting that as in Clostridium sp. these sequences could mediate the binding of enzymatic polypeptides to another component in the cell surface enzyme complex of R. flavefaciens.

Phenotypic diversity among ruminal isolates of Prevotella ruminicola: proposal of Prevotella brevis sp. nov., Prevotella bryantii sp. nov., and Prevotella albensis sp. nov. and redefinition of Prevotella ruminicola

Selected phenotypic characteristics of isolates of Prevotella ruminicola (formerly Bacteroides ruminicola) were studied in order to establish whether the characteristics of genotypic strain groups established previously on the basis of 16S ribosomal DNA sequences differed systematically. Among strains formerly considered P. ruminicola subsp. brevis, strains related to strain GA33T (T = type strain) typically failed to produce carboxymethyl cellulase (CMCase) activity detectable by plate assays and failed to ferment xylose, while strains related to strain B14T produced abundant CMCase and fermented xylose. We propose that strains related to GA33T, which have DNA G + C contents between 45 and 51 mol%, should be assigned to a new species, Prevotella brevis, and that strains related to B14T, which have DNA G + C contents between 39 and 43 mol%, should be assigned to another new species, Prevotella bryantii. Most of the isolates formerly classified as P. ruminicola subsp. ruminicola strains produced CMCase and had DNA G + C contents between 45 and 51 mol%, and we propose that these organisms should be placed in the redefined species P. ruminicola. A small group of isolates that have lower G + C contents are assigned to another new species, Prevotella albensis. Most P. brevis and P. bryantii strains produced abundatn extracellular DNase activity. Proteinase activities (as determined by [14C]casein hydrolysis) varied widely between strains, and P. brevis strains exhibited the highest mean activity. All strains produced dipeptidyl peptidase activity, but the relative activities against different peptide substrates exhibited by P. bryantii, P. albensis, and P. brevis differed systematically. The phenotypic differences among the newly defined species suggest that they may occupy distinct niches within the rumen ecosystem.

Interrupted catalytic domain structures in xylanases from two distantly related strains of Prevotella ruminicola

Two xylanases from the rumen anaerobic bacterium Prevotella ruminicola were found to possess highly unusual structures in which family 10 catalytic domains are interrupted by unrelated sequences. XynC from P. ruminicola B(1)4 carries a 160 amino-acid insertion, while a P. ruminicola D31d xylanase carries an unrelated region of 280 amino acids, containing an imperfect 130 amino-acid duplication. Both regions of family 10 similarity were shown to be essential for activity of the D31d enzyme.

Genetic transfer of lactate-utilizing ability in the rumen bacterium Selenomonas ruminantium

Matings between the lactate-utilizing, tetracycline-sensitive Selenomonas ruminantium strains 5521C1 and 5934e and the lactate-non-utilizing, tetracycline-resistant strain FB322 resulted in putative recombinant strains capable of growth on lactate. Analysis of total protein by SDS-PAGE and chromosomal DNA by hybridization, indicated that the recombinants were derived from strain FB322. DNA hybridization produced no evidence that plasmid transfer occurred, leaving chromosomal DNA transfer as the most likely mechanism for the altered phenotype. Analysis of strains 5934e, FB322 and the resulting recombinant TC3 indicated that all three strains contained D-nLDH and L-nLDH activities. In addition strains 5934e and TC3 possessed D-iLDH activity when grown on DL-lactate. The ability of strain FB322 to grow on pyruvate but not lactate suggested that the lactate-utilizing recombinant had acquired the ability to synthesize D-iLDH.

Expression of a cloned cellulase/xylanase gene from Prevotella ruminicola in Bacteroides vulgatus, Bacteroides uniformis and Prevotella ruminicola

A new shuttle vector, pRH3 (8.7 kb), was constructed for use in Prevotella/Bacteroides host strains. This vector combines the pRRI2 replicon from P. ruminicola, pBluescript sequences and a tetQ marker gene for selection in Prevotella/Bacteroides hosts. Following insertion of a fragment carrying an endoglucanase/xylanase gene from P. ruminicola 23 into the multiple cloning site, the resulting construct, pRH3X, was introduced into B. vulgatus 1447, B. uniformis 1100 and P. ruminicola 2202. This resulted in increases of between 4 and 50-fold in CM-cellulase and xylanase activities in cells grown with glucose. In contrast activities were barely detectable for the same construct in E. coli DH5 alpha. Most of the total xylanase activity produced was found within the cell in P. ruminicola 2202 and B. vulgatus 1447 transformed with pRH3X, and in P. ruminicola 23. An osmotic shock experiment indicated that a significant proportion of the xylanase activity in B. vulgatus 1447 cells carrying pRH3X was periplasmic.

A xylan hydrolase gene cluster in Prevotella ruminicola B(1)4: sequence relationships, synergistic interactions, and oxygen sensitivity of a novel enzyme with exoxylanase and beta-(1,4)-xylosidase activities

Two genes concerned with xylan degradation were found to be closely linked in the ruminal anaerobe Prevotella ruminicola B(1)4, being separated by an intergenic region of 75 nucleotides. xynA is shown to encode a family F endoxylanase of 369 amino acids, including a putative amino-terminal signal peptide. xynB encodes an enzyme of 319 amino acids, with no obvious signal peptide, that shows 68% amino acid identity with the xsa product of Bacteroides ovatus and 31% amino acid identity with a beta-xylosidase from Clostridium stercorarium; together, these three enzymes define a new family of beta-(1,4)-glycosidases. The activity of the cloned P. ruminicola xynB gene product, but not that of the xynA gene product, shows considerable sensitivity to oxygen. Studied under anaerobic conditions, the XynB enzyme was found to act as an exoxylanase, releasing xylose from substrates including xylobiose, xylopentaose, and birch wood xylan, but was relatively inactive against oat spelt xylan. A high degree of synergy (up to 10-fold stimulation) was found with respect to the release of reducing sugars from oat spelt xylan when XynB was combined with the XynA endoxylanase from P. ruminicola B(1)4 or with endoxylanases from the cellulolytic rumen anaerobe Ruminococcus flavefaciens 17. Pretreatment with a fungal arabinofuranosidase also stimulated reducing-sugar release from xylans by XynB. In P. ruminicola the XynA and XynB enzymes may act sequentially in the breakdown of xylan.

Heterologous expression of an endoglucanase gene (endA) from the ruminal anaerobe Ruminococcus flavefaciens 17 in Streptococcus bovis and Streptococcus sanguis

The heterologous expression of a cloned endoglucanase gene (endA) from the ruminal bacterium Ruminococcus flavefaciens 17 was demonstrated in the Streptococcus species S. bovis JB1 and S. sanguis DL1. The endA gene was introduced into S. bovis and S. sanguis using the Escherichia coli/Streptococcus shuttle vector pVA838. Expression of the gene was detected by clearing zones around the recombinant colonies on agar plates containing carboxymethylcellulose stained with Congo red. S. bovis JB1 containing the endA gene was capable of utilizing cellotetraose at a faster rate than the parent strain. This is the first demonstration that Streptococcus species can express a gene from a Ruminococcus flavefaciens strain.

Transfer of plasmids between strains of Escherichia coli under rumen conditions

Strains of Escherichia coli originally isolated from the rumen of sheep were shown to be capable of exchanging a 60kb plasmid, conferring resistance to tetracycline and ampicillin, at low frequencies (below 10(-6) per recipient) under anaerobic conditions in the presence of (a) autoclaved and clarified rumen fluid, (b) raw clarified rumen fluid, or (c) whole rumen fluid. Under anaerobic conditions the two rumen strains showed no inhibition of growth rate when 50 mmol l-1 volatile fatty acids were added to LB medium at pH 7, although significant inhibition resulted with 100 mmol l-1 VFA. The two rumen strains, and four strains from the pig gut, showed less inhibition of anaerobic growth by volatile fatty acids than did three laboratory strains examined for comparison. These findings indicate that plasmid transfer between certain E. coli strains can occur under conditions that closely simulate an anaerobic but environment.

Isolation of genes encoding beta-D-xylanase, beta-D-xylosidase and alpha-L-arabinofuranosidase activities from the rumen bacterium Prevotella ruminicola B1(4)

Prevotella ruminicola B1(4) is a strictly anaerobic, Gram-negative, polysaccharide-degrading rumen bacterium. Xylanase activity in this strain was found to be inducible, the specific activity of cells grown on xylan being increased at least 20-fold by comparison with cells grown on glucose. Ten bacteriophage clones expressing xylanase activity were isolated from a lambda EMBL3 genomic DNA library of P. ruminicola B1(4). These clones were shown to represent four distinct chromosomal regions, based on restriction enzyme analysis and DNA hybridisation. Three groups of clones encoded activity against oat spelt xylan but not carboxymethylcellulose (CMC). In one of these groups, represented by clone 5, activities against pNP-arabinofuranoside and pNP-xyloside were found to be encoded separately from endoxylanase activity. The fourth region encoded activity against CM cellulose and lichen, in addition to xylan, and contains an endoglucanase/xylanase gene isolated previously.

Identification of non-catalytic conserved regions in xylanases encoded by the xynB and xynD genes of the cellulolytic rumen anaerobe Ruminococcus flavefaciens

xynB is one of at least four genes from the cellulolytic rumen anaerobe Ruminococcus flavefaciens 17 that encode xylanase activity. The xynB gene is predicted to encode a 781-amino acid product starting with a signal peptide, followed by an amino-terminal xylanase domain which is identical at 89% and 78% of residues, respectively, to the amino-terminal xylanase domains of the bifunctional XynD and XynA enzymes from the same organism. Two separate regions within the carboxy-terminal 537 amino acids of XynB also show close similarities with domain B of XynD. These regions show no significant homology with cellulose- or xylan-binding domains from other species, or with any other sequences, and their functions are unknown. In addition a 30 to 32-residue threonine-rich region is present in both XynD and XynB. Codon usage shows a consistent pattern of bias in the three xylanase genes from R. flavefaciens that have been sequenced.

Molecular genetics of obligate anaerobes from the rumen

The rumen is inhabited by a highly specialised microflora consisting of obligately anaerobic bacteria, fungi and protozoa. Rumen bacteria belong to many different phylogenetic groupings and many species exhibit a high degree of rRNA gene sequence diversity, whereas the rumen fungi are monophyletic. At least 21 genes concerned with the degradation and utilisation of plant cell wall polysaccharides, from five species of rumen bacteria and from rumen fungi, have been isolated and sequenced. In general, the catalytic domains of the encoded enzymes belong to enzyme families identified among non-rumen microorganisms, but some show unusual organisation, consisting of multiple catalytic domains. Several bacterial species have been used as recipients for gene transfer by electrotransformation or by conjugation, allowing development of methods for genetic analysis. The rumen is also considered as a potential site for natural gene transfer.

Multiple lactate dehydrogenase activities of the rumen bacterium Selenomonas ruminantium

The lactate utilizing strain of Selenomonas ruminantium 5934e was found to contain three lactate dehydrogenase (LDH) activities in sonicated cell extracts. One activity, an NAD dependent L-LDH (L-nLDH) was measured at 15-fold greater levels in extracts of cells grown to mid-exponential phase on glucose compared to cells grown to the equivalent growth stage on DL-lactate. A second nLDH activity specific for D-lactate (D-nLDH) was detected at similar levels in both lactate-grown cell extracts and glucose-grown cell extracts. The third activity, an NAD independent DLDH (D-iLDH) was very low in cells grown on glucose but was induced more than 10-fold when DL-lactate was used as the carbon source. The three LDH activities could be separated by gel filtration. Recovery of the activities was low due to the apparent instability of the enzymes at 4 degrees C, which was most pronounced in the case of the D-iLDH. A Km for lactate of 0.5 mM was estimated for the D-iLDH and this was considerably lower than the values of 45 mM and 70 mM measured for L-nLDH and D-nLDH respectively. It is proposed that the D-iLDH may be largely responsible for the formation of pyruvate in lactate-grown cells of S. ruminantium strain 5934e. Three other lactate utilizing strains of S. ruminantium, HD4, 5521C1 and JW13 exhibited a similar profile of LDH activities to strain 5934e when grown on glucose and DL-lactate.

Genetic diversity and phylogenetic relationships among strains of Prevotella (Bacteroides) ruminicola from the rumen

A high degree of genetic diversity among 29 strains of Prevotella (Bacteroides) ruminicola from the rumen was revealed by comparing restriction fragment length polymorphisms in 16S rRNA genes, sodium dodecyl sulfate-polyacrylamide gel profiles of total-cell proteins, and G + C contents of chromosomal DNAs. In order to obtain information on phylogenetic relationships, the sequences of a 389-bp region of the 16S rRNA gene, including variable regions 4 and 5, were compared for 10 strains. These 10 strains formed a single group when their sequences were compared with 16S ribosomal DNA sequences from other species, including Bacteroides spp. from the human colon. On the other hand, the great genetic distances between many P. ruminicola strains, including P. ruminicola subsp. brevis B(1)4 and GA33 and P. ruminicola 23T (T = type strain), support the hypothesis that these organisms should be reclassified into new species. We identified signature oligonucleotides based on 16S ribosomal DNA sequences that distinguished strains related to strains 23T, B(1)4, GA33, and M384, as well as an oligonucleotide that specifically recognized all but one of the Bacteroides and Prevotella strains tested. On the basis of the priming activities of these signature oligonucleotides in PCR reactions and on other criteria, we concluded that 12 of the original 29 strains were related to strain 23T, 4 were related to strain B(1)4, and 4 were related to strain GA33. While there are clear grounds for subdividing the species P. ruminicola on the basis of genotypic differences, it is appropriate to delay formal reclassification until further work on the phenotypic differentiation of the new groups is completed.

Mode of action, kinetic properties and physicochemical characterization of two different domains of a bifunctional (1-->4)-beta-D-xylanase from Ruminococcus flavefaciens expressed separately in Escherichia coli

Two catalytic domains, A and C, of xylanase A (XYLA) from Ruminococcus flavefaciens were expressed separately as truncated gene products from lacZ fusions in Escherichia coli. The fusion products, referred to respectively as XYLA-A1 and XYLA-C2, were purified to homogeneity by anion-exchange chromatography and chromatofocusing. XYLA-A1 was isoelectric at pH 5.0 and had a molecular mass of 30 kDa, whereas XYLA-C2 had a pI of 5.4 and a molecular mass of 44 kDa. The catalytic activity shown by both domains was optimal at 50 degrees C, but XYLA-A1 was more sensitive than XYLA-C2 to temperatures higher than the optimum. XYLA-A1 showed a higher sensitivity to pH than XYLA-C2. The enzyme activity of both domains was completely inactivated in the presence of copper or silver ions and partially inactivated by iron or zinc ions. Neither domain was active on xylo-oligosaccharides shorter than xylopentaose: the rate of degradation of longer xylo-oligosaccharides (degree of polymerization 5-10) increased as the chain length increased. Analysis of the products of hydrolysis of xylo-oligosaccharides and xylan (arabinoxylan) polysaccharide showed that the two domains differed in their modes of action: xylobiose was the shortest product of the hydrolysis. With oat spelt xylan as substrate, XYLA-A1 activity was apparently restricted to regions where xylopyranosyl residues did not carry arabinofuranosyl substituents, whereas XYLA-C2 was able to release hetero-oligosaccharides carrying arabinofuranosyl residues. Neither domain was able to release arabinose from oat spelt xylan.

A bifunctional enzyme, with separate xylanase and beta(1,3-1,4)-glucanase domains, encoded by the xynD gene of Ruminococcus flavefaciens

Adjacent regions of a Ruminococcus flavefaciens 17 DNA fragment were found to encode xylanase and beta(1,3-1,4)-glucanase activities. Sequencing of this fragment showed that both activities are encoded by a single 2,406-bp open reading frame corresponding to the xynD gene. The predicted product has a characteristic signal sequence that is followed by an amino-terminal domain related to family G xylanases, while the carboxyterminal domain is related to beta(1,3-1,4)-glucanases from several other bacterial species. These two domains are connected by a region of unknown function that consists of 309 amino acids and includes a 30-amino-acid threonine-rich sequence. A polypeptide having a molecular weight of approximately 90,000 and exhibiting xylanase and beta(1,3-1,4)-glucanase activities was detected in Escherichia coli cells carrying the cloned xynD gene. This is one of the first cases in which a microbial polysaccharidase has been shown to carry separate catalytic domains active against different plant cell wall polysaccharides within the same polypeptide. xynD is one of a family of related genes in R. flavefaciens that encode enzymes having multiple catalytic domains, and the amino terminus of XYLD exhibits a high degree of similarity with the corresponding regions of another xylanase, XYLA, which carries two different xylanase catalytic domains.

Transfer of hybrid plasmids based on the replicon pRRI7 from Escherichia coli to Bacteroides and Prevotella strains

New shuttle vectors based on a Prevotella ruminicola 9.5 kb cryptic plasmid (pRRI7) inserted within the Escherichia coli vector pKC71, carrying the Ccr/Emr Bacteroides marker, were constructed. These constructs (pKBR23-1 and pKBR23-2) were transferred into Bacteriodes distasonis, Bacteroides thetaiotaomicron, Bacteroides uniformis and into P. ruminicola NCFB 2202 either by conjugal mobilization or by electroporation. Another pRRI7 derivative based on pKC72, pKBR23-3, was smaller (13.1 kb) and non-mobilizable. By electroporation, it was transferred to Bact. distasonis and P. ruminicola. Being derived from pRRI7 which is compatible with the shuttle plasmid pRRI207, the host/vector combination involving P. ruminicola NCFB 2202 and pKBR23-3 offers new possibilities for genetic investigations in rumen anaerobic bacteria after further introduction of a second readily selectable marker within pRRI207 or pKBR23-3.