Molecular cloning of Bacillus polymyxa (1→4)-β-d-xylanase gene in Escherichia coli

Cloning and heterologous expression of cellulose free thermostable xylanase from Bacillus brevis

Xylanase gene isolated from Bacillus brevis was expressed in E. coli BL21. Sequencing of the gene (Gen Bank accession number: HQ179986) showed that it belongs to family 11 xylanases. The recombinant xylanase was predominantly secreted to culture medium and showed mesophilic nature (optimum activity at 55°C and pH 7.0). The cell free culture medium exhibited 30 IU/ml xylanse activity. The enzyme did not show any cellulose activity and was active under wide range of temperature (40°C to 80°C) and pH (4 to 9). The enzyme showed considerable thermo stability and regained over 90% of activity, when returned to 55°C after boiling for 5 min. These physiochemical properties of B. brevis xylanse show high potential of its applications in paper and pulp industry.

Molecular and biotechnological aspects of xylanases

Hemicellulolytic microorganisms play a significant role in nature by recycling hemicellulose, one of the main components of plant polysaccharides. Xylanases (EC 3.2.1.8) catalyze the hydrolysis of xylan, the major constituent of hemicellulose. The use of these enzymes could greatly improve the overall economics of processing lignocellulosic materials for the generation of liquid fuels and chemicals. Recently cellulase-free xylanases have received great attention in the development of environmentally friendly technologies in the paper and pulp industry. In microorganisms that produce xylanases low molecular mass fragments of xylan and their positional isomers play a key role in regulating its biosynthesis. Xylanase and cellulase production appear to be regulated separately, although the pleiotropy of mutations, which causes the elimination of both genes, suggests some linkage in the synthesis of the two enzymes. Xylanases are found in a cornucopia of organisms and the genes encoding them have been cloned in homologous and heterologous hosts with the objectives of overproducing the enzyme and altering its properties to suit commercial applications. Sequence analyses of xylanases have revealed distinct catalytic and cellulose binding domains, with a separate non-catalytic domain that has been reported to confer enhanced thermostability in some xylanases. Analyses of three-dimensional structures and the properties of mutants have revealed the involvement of specific tyrosine and tryptophan residues in the substrate binding site and of glutamate and aspartate residues in the catalytic mechanism. Many lines of evidence suggest that xylanases operate via a double displacement mechanism in which the anomeric configuration is retained, although some of the enzymes catalyze single displacement reactions with inversion of configuration. Based on a dendrogram obtained from amino acid sequence similarities the evolutionary relationship between xylanases is assessed. In addition the properties of xylanases from extremophilic organisms have been evaluated in terms of biotechnological applications.

Purification and Characterization of Alkaline Xylanases from Bacillus polymyxa

By applying different classical and fast protein liquid chromatographic techniques, three xylanases (β-1,4- d -xylan xylanhydrolase) were purified to homogeneity from the extracellular enzymatic complex of Bacillus polymyxa . The three enzymes (X 34 C, X 34 E, and X 22 ) were small proteins of 34, 34, and 22 kDa and basic pIs 9.3, >9.3, and 9.0, respectively. X 34 C and X 34 E are closely related and seem to be isoforms of the same enzyme. However, they differ in some characteristics. The three enzymes had different pH and temperature optima. One of them, X 34 E, showed a high thermal stability. The V max values determined for X 34 C, X 34 E, and X 22 enzymes on oat spelts xylan were 14.9, 85.5, and 64.0 U mg -1 , respectively, and 16.1, 62.0, and 150.6 U mg -1 on birchwood xylan. When oat spelts xylan was the substrate used, K m values of 3.4, 2.4, and 1.9 mg ml -1 were obtained for X 34 C, X 34 E, and X 22 enzymes, respectively, and 0.65, 6.3, and 0.32 mg ml -1 were the respective K m values determined with birchwood xylan as the substrate. The enzymes were nondebranching endo-β-xylanases. Xylose was one of the products of xylan hydrolysis by xylanases X 34 C and X 34 E, but this monosaccharide was not released by X 22 enzyme. However, neither of the enzymes was able to degrade xylobiose.

Cloning and extracellular expression in Escherichia coli of xylanases from an alkaliphilic thermophilic Bacillus sp. NCIM 59

A genomic DNA library of an alkaliphilic thermophilic Bacillus was constructed in Escherichia coli with pUC 8 vector and was screened using a Congo red xylan plate clearance assay. Six xylanase positive transformants having identical inserts showed immunological reactivity towards polyclonal antibodies raised against purified xylanase (M(r) 15,800) from the Bacillus. A 4.5-kb HindIII-EcoRI subfragment was found to code for two xylanases of M(r) 14,500 and 35,000. Equivalent amounts of xylanase activity were detected from IPTG induced and noninduced recombinants irrespective of the orientation of the 4.5-kb insert with respect to the lac promoter, indicating that xylanase gene expression was under the control of its own promoter. 95% of the xylanase activity (2 U/ml) was found in the extracellular culture filtrate. The hydrolysis of xylan by the recombinant xylanases yielded mainly xylobiose.

Transformation of Bacillus polymyxa with plasmid DNA

A plasmid transformation system was developed for Bacillus polymyxa ATCC 12321 and derivatives of this strain. The method utilizes a penicillin-treated-cell technique to facilitate uptake of the plasmid DNA. Low-frequency transformation (10(-6) per recipient cell) of plasmids pC194, pBD64, and pBC16 was accomplished with this method. Selection for the transformants was accomplished on both hypertonic and nonhypertonic selective media, with the highest rates of recovery occurring on a peptone-glucose-yeast extract medium containing 0.25 M sucrose. Several additional plasmids were shown to be capable of transferring their antibiotic resistance phenotypes to B. polymyxa through the use of a protoplast transformation procedure which allowed for a more efficient transfer of the plasmid DNA. However, cell walls could not be regenerated on the transformed protoplasts, and the transformants could not be subcultured from the original selective media.

Cloning of a Thermomonospora fusca xylanase gene and its expression in Escherichia coli and Streptomyces lividans

Thermomonospora fusca chromosomal DNA was partially digested with EcoRI to obtain 4- to 14-kilobase fragments, which were used to construct a library of recombinant phage by ligation with EcoRI arms of lambda gtWES. lambda B. A recombinant phage coding for xylanase activity which contained a 14-kilobase insert was identified. The xylanase gene was localized to a 2.1-kilobase SalI fragment of the EcoRI insert by subcloning onto pBR322 and derivatives of pBR322 that can also replicate in Streptomyces lividans. The xylanase activity produced by S. lividans transformants was 10- to 20-fold higher than that produced by Escherichia coli transformants but only one-fourth the level produced by induced T. fusca. A 30-kilodalton peptide with activity against both Remazol brilliant blue xylan and xylan was produced in S. lividans transformants that carried the 2.1-kilobase SalI fragment of T. fusca DNA and was not produced by control transformants. T. fusca cultures were found to contain a xylanase of a similar size that was induced by growth on xylan or Solka Floc. Antiserum directed against supernatant proteins isolated from a Solka Floc-grown T. fusca culture inhibited the xylanase activity of S. lividans transformants. The cloned T. fusca xylanase gene was expressed at about the same level in S. lividans grown in minimal medium containing either glucose, cellobiose, or xylan. The xylanase bound to and hydrolyzed insoluble xylan. The cloned xylanase appeared to be the same as the major protein in xylan-induced T. fusca culture supernatants, which also contained at least three additional minor proteins with xylanase activity and having apparent molecular masses of 43, 23, and 20 kilodaltons.

Identification of two distinct Bacillus circulans xylanases by molecular cloning of the genes and expression in Escherichia coli

Two genes coding for xylanase synthesis in Bacillus circulans were cloned and expressed in Escherichia coli. After digestion of genomic DNA from Bacillus circulans with EcoRI and PstI, the fragments were ligated into the corresponding sites of pUC19 and transformed into Escherichia coli. Restriction enzyme mapping of the two inserts coding for xylanase activity indicated distinctly different nucleotide sequences. Cross-hybridization assays confirmed the absence of sequence homology between the two genes. In vitro transcription-translation assays indicated that the cloned genes encoded for proteins with molecular weights of 22,000 and 59,000. The gene products displayed different substrate specificities. The 22,000-dalton enzyme readily hybrolyzed aspeen, larchwood, and oat spelt xylans, whereas the second was unable to extensively depolymerize oat spelt xylan and resulted in very limited reducing sugar release from any of the xylan substrates tested. Both of the xylanases had isoelectric points of approximately 9.0.

Molecular Cloning and Expression of a Xylanase Gene from Bacillus polymyxa in Escherichia coli

Genomic fragments of Bacillus polymyxa derived from separate and complete digestion by Eco RI, Hin dIII, and Bam HI were ligated into the corresponding sites of pBR322, and the resulting chimeric plasmids were transformed into Escherichia coli. Of 6,000 transformants screened, 1 (pBPX-277) produced a clear halo on Remazol brilliant blue xylan plates. The insert in the pBPX-277 recombinant, identified as an 8.0-kilobase Bam HI fragment of B. polymyxa , was subsequently subjected to extensive mapping and a series of subclonings into pUC19. A 2.9-kilobase Bam HI- Eco RI subfragment was found to code for xylanase activity. Xylanase activity expressed by E. coli harboring the cloned gene was located primarily in the periplasm and corresponded to one of two distinct xylanases produced by B. polymyxa. Xylanase expression by the cloned gene occurred in the absence of xylan and was reduced by glucose and xylose. Southern blot hybridization with the cloned fragment as a probe against complete genomic digests of the bacilli B. polymyxa, B. circulans , and B. subtilis revealed that the cloned xylanase gene was unique to B. polymyxa. The xylanase expressed by the cloned gene had a molecular weight of approximately 48,000 and an isoelectric point of 4.9.

Structure and expression of genes coding for xylan-degrading enzymes of Bacillus pumilus

The complete nucleotide sequence of the beta-xylosidase gene (xynB) of Bacillus pumilus IPO and its flanking regions was established. A 1617-bp open reading frame for beta-xylosidase, a homodimer enzyme, was observed. The amino acid sequence of the N-terminal region and the molecular mass 62607 Da) of the beta-xylosidase subunit, deduced from the DNA sequence, agreed with the result obtained with the purified enzyme. The Shine-Dalgarno sequence was found 8 bp upstream of the initiation codon, ATG. The xylanase gene (xynA) of the same strain was 4.6 kbp downstream of the 3' end of xynB, and its DNA sequence was reported in our previous paper [Fukusaki, E., Panbangred, W., Shinmyo, A. & Okada, H. (1984) FEBS Lett. 171, 197-201]. The results of the Northern hybridization suggested that the mRNA of xynA and xynB were produced separately. The 5' and 3' ends of the xynA and xynB gene were mapped with nuclease S1. The '-10' regions for promoter sequences of both genes were similar to the consensus sequence for B. subtilis RNA polymerases, the '-35' regions were different from all the known promoters for B. subtilis RNA polymerases.

Cloning of the Bacillus subtilis DLG beta-1,4-glucanase gene and its expression in Escherichia coli and B. subtilis

The gene encoding beta-1,4-glucanase in Bacillus subtilis DLG was cloned into both Escherichia coli C600SF8 and B. subtilis PSL1, which does not naturally produce beta-1,4-glucanase, with the shuttle vector pPL1202. This enzyme is capable of degrading both carboxymethyl cellulose and trinitrophenyl carboxymethyl cellulose, but not more crystalline cellulosic substrates (L. M. Robson and G. H. Chambliss, Appl. Environ. Microbiol. 47:1039-1046, 1984). The beta-1,4-glucanase gene was localized to a 2-kilobase (kb) EcoRI-HindIII fragment contained within a 3-kb EcoRI chromosomal DNA fragment of B. subtilis DLG. Recombinant plasmids pLG4000, pLG4001a, pLG4001b, and pLG4002, carrying this 2-kb DNA fragment, were stably maintained in both hosts, and the beta-1,4-glucanase gene was expressed in both. The 3-kb EcoRI fragment apparently contained the beta-1,4-glucanase gene promoter, since transformed strains of B. subtilis PSL1 produced the enzyme in the same temporal fashion as the natural host B. subtilis DLG. B. subtilis DLG produced a 35,200-dalton exocellular beta-1,4-glucanase; intracellular beta-1,4-glucanase was undetectable. E. coli C600SF8 transformants carrying any of the four recombinant plasmids produced two active forms of beta-1,4-glucanase, an intracellular form (51,000 +/- 900 daltons) and a cell-associated form (39,000 +/- 400 daltons). Free exocellular enzyme was negligible. In contrast, B. subtilis PSL1 transformed with recombinant plasmid pLG4001b produced three distinct sizes of active exocellular beta-1,4-glucanase: approximately 36,000, approximately 35,200, and approximately 33,500 daltons. Additionally, B. subtilis PSL1(pLG4001b) transformants contained a small amount (5% or less) of active intracellular beta-1,4-glucanase of three distinct sizes: approximately 50,500, approximately 38,500 and approximately 36,000 daltons. The largest form of beta-1,4-glucanase seen in both transformants may be the primary, unprocessed translation product of the gene.