Conserved reiterated domains in Clostridium thermocellum endoglucanases are not essential for catalytic activity

The non-catalytic C-terminal region of endoglucanase E from Clostridium thermocellum contains a cellulose-binding domain

Mature endoglucanase E (EGE) from Clostridium thermocellum consists of 780 amino acid residues and has an Mr of 84,016. The N-terminal 334 amino acids comprise a functional catalytic domain. Full-length EGE bound to crystalline cellulose (Avicel) but not to xylan. Bound enzyme could be eluted with distilled water. The capacity of truncated derivatives of the enzyme to bind cellulose was investigated. EGE lacking 109 C-terminal residues (EGEd) or a derivative in which residues 367-432 of the mature form of the enzyme had been deleted (EGEb), bound to Avicel, whereas EGEa and EGEc, which lack 416 and 246 C-terminal residues respectively, did not. The specific activity of EGEa, consisting of the N-terminal 364 amino acids, was 4-fold higher than that of the full-length enzyme. The truncated derivative also exhibited lower affinity for the substrate beta-glucan than the full-length enzyme. It is concluded that EGE contains a cellulose-binding domain, located between residues 432 and 671, that is distinct from the active site. The role of this substrate-binding domain is discussed.

Transcription of Clostridium thermocellum endoglucanase genes celF and celD

Transcripts of the Clostridium thermocellum endoglucanase genes celF and celD, encoding endoglucanases F and D, respectively, were characterized. The size of the mRNAs was about 2.35 kb for celF and 2.1 kb for celD, indicating monocistronic transcription of both genes. A unique 5' end, located 218 bp upstream from the initiation codon, was found for celF mRNA. No convincing homology could be identified between the sequence upstream from the celF 5' end and other procaryotic promoters. Two 5' ends, located 124 and 294 bp upstream from the initiation codon, were mapped for celD mRNA. The -10 and the -35 sequences preceding the ATG-distal 5' end of celD mRNA were homologous to the consensus sequence of Bacillus subtilis sigma 43 promoters. The sequence upstream from the ATG-proximal 5' end had some similarity with the -10 sequence of B. subtilis sigma 28 promoters. During growth on cellobiose, the 5' end of celD transcripts was found predominantly at the -124 site during the late exponential phase but almost exclusively at the -294 site during the early stationary phase. The kinetics of appearance of celA, celC, celD, and celF mRNA was followed by dot blot analysis. Transcripts of celA, celD, and celF were detected during late exponential and early stationary phase. In contrast, the celC transcript was detected almost exclusively during early stationary phase. Since growth was limited by the availability of cellobiose, the results suggest that the genes are regulated by a mechanism analogous to catabolite repression.

Xylanase B and an arabinofuranosidase from Pseudomonas fluorescens subsp. cellulosa contain identical cellulose-binding domains and are encoded by adjacent genes

The complete nucleotide sequence of the Pseudomonas fluorescens subsp. cellulosa xynB gene, encoding an endo-beta-1,4-xylanase (xylanase B; XYLB) has been determined. The structural gene consists of an open reading frame (ORF) of 1775 bp coding for a protein of Mr 61,000. A second ORF (xynC) of 1712 bp, which starts 148 bp downstream of xynB, encodes a protein, designated xylanase C (XYLC), of Mr 59,000. XYLB hydrolyses oat spelt xylan to xylobiose and xylose, whereas XYLC releases only arabinose from the same substrate. Thus XYLB is a typical xylanase and XYLC is an arabinofuranosidase. Both enzymes bind to crystalline cellulose (Avicel), but not to xylan. The nucleotide sequences between residues 114 and 931 of xynB and xynC were identical, as were amino acid residues 39-311 of XYLB and XYLC. This conserved sequence is reiterated elsewhere in the P. fluorescens subsp. cellulosa genome. Truncated derivatives of XYLB and XYLC, in which the conserved sequence had been deleted, retained catalytic activity, but did not exhibit cellulose binding. A hybrid gene in which the 5' end of xynC, encoding residues 1-110 of XYLC, was fused to the Escherichia coli pho A' gene (encodes mature alkaline phosphatase) directed the synthesis of a fusion protein which exhibited alkaline phosphatase activity and bound to cellulose.

Recent advances in rumen microbial ecology and metabolism: potential impact on nutrient output

Feedstuffs consumed by ruminants are all initially exposed to fermentative activity in the rumen prior to gastric and intestinal digestion. The extent and type of transformation of feedstuffs thus determines the productive performance of the host. Research on rumen microbial ecology and metabolism is essentially a study of the interactions between the host, microorganisms present, substrates available, and end products of digestion. Furthermore, the interactions of the normal microbial flora with the host can be manipulated to improve the efficiency of nutrient utilization in ruminant animals. Three important areas of ruminal fermentation will be reviewed, N metabolism, fiber degradation, and biotransformation of toxic compounds. The extent of protein degradation and the rate of uptake of resultant peptides and ammonia are extremely important factors in determining the efficiency of N utilization by rumen bacteria and, therefore, the relative amounts of microbial or bypass protein available to the host. Strategies aimed at identifying and characterizing rate-limiting enzymes of cellulolytic bacteria are essential in elucidating mechanisms involved in ruminal fiber degradation. Results obtained with ruminococci will be described. The detoxification of phytotoxins by passage through the gastrointestinal tract of ruminants is a process deserving special attention and several examples will be presented. Opportunities for manipulation of rumen fermentation are good. However, successful manipulation and full exploitation depend on a through understanding of the mechanisms involved.

celB, a gene coding for a bifunctional cellulase from the extreme thermophile "Caldocellum saccharolyticum"

"Caldocellum saccharolyticum" is an obligatory anaerobic thermophilic bacterium. A gene from this organism, designated celB, has been cloned in Escherichia coli as part of a bacteriophage lambda gene library. This gene produces a thermostable cellulase that shows both endoglucanase and exoglucanase activities on test substrates and is able to degrade crystalline cellulose to glucose. The sequence of celB has homology with both exo- and endoglucanases described by others. It appears to have a central domain without enzymatic activity which is joined to the enzymatic domains by runs of amino acids rich in proline and threonine (PT boxes). Deletion analysis shows that the exoglucanase activity is located in the amino-terminal domain of the enzyme and that endoglucanase activity is located in the carboxy-terminal domain. There are internal transcriptional and translational start sites within the gene. The intact gene has been cloned into a temperature-inducible expression vector, pJLA602, and overexpressed in E. coli. Polyacrylamide gel electrophoresis showed that celB produced a protein with a molecular weight of 118,000 to 120,000. A number of smaller proteins with activity against carboxymethyl cellulose and 4-methyl umbelliferyl-beta-D-cellobioside were also produced. These are believed to be the result of alternative translational start sites and/or proteolytic degradation products of the translated gene product.

Sequencing and expression of a cellodextrinase (ced1) gene from Butyrivibrio fibrisolvens H17c cloned in Escherichia coli

The nucleotide sequence of a 2.314 kb DNA segment containing a gene (ced1) expressing cellodextrinase activity from Butyrivibrio fibrisolvens H17c was determined. The B. fibrisolvens H17c gene was expressed from a weak internal promoter in Escherichia coli and a putative consensus promoter sequence was identified upstream of a ribosome binding site and a GTG start codon. The complete amino acid sequence (547 residues) was deduced and homology was demonstrated with the Clostridium thermocellum endoglucanase D (EGD), Pseudomonas fluorescens var. cellulosa endoglucanase (EG), and a cellulase from the avocado fruit (Persea americana). The ced1 gene product Ced1 showed cellodextrinase activity and rapidly hydrolysed short-chain cellodextrins to yield either cellobiose or cellobiose and glucose as end products. The Ced1 enzyme released cellobiose from p-nitrophenyl-beta-D-cellobioside and the enzyme was not inhibited by methylcellulose, an inhibitor of endoglucanase activity. Although the major activity of the Ced1 enzyme was that of a cellodextrinase it also showed limited activity against endoglucanase specific substrates [carboxymethylcellulose (CMC), lichenan, laminarin and xylan]. Analysis by SDS-polyacrylamide gel electrophoresis with incorporated CMC showed a major activity band with an apparent Mr of approximately 61,000. The calculated Mr of the ced1 gene product was 61,023.

Sequence analysis of the Clostridium stercorarium celZ gene encoding a thermoactive cellulase (Avicelase I): identification of catalytic and cellulose-binding domains

The nucleotide sequence of the celZ gene coding for a thermostable endo-beta-1,4-glucanase (Avicelase I) of Clostridium stercorarium was determined. The structural gene consists of an open reading frame of 2958 bp which encodes a preprotein of 986 amino acids with an Mr of 109,000. The signal peptide cleavage site was identified by comparison with the N-terminal amino acid sequence of Avicelase I purified from C. stercorarium culture supernatants. The recombinant protein expressed in Escherichia coli is proteolytically cleaved into catalytic and cellulose-binding fragments of about 50 kDa each. Sequence comparison revealed that the N-terminal half of Avicelase I is closely related to avocado (Persea americana) cellulase. Homology is also observed with Clostridium thermocellum endoglucanase D and Pseudomonas fluorescens cellulase. The cellulose-binding region was located in the C-terminal half of Avicelase I. It consists of a reiterated domain of 88 amino acids flanked by a repeated sequence about 140 amino acids in length. The C-terminal flanking sequence is highly homologous to the non-catalytic domain of Bacillus subtilis endoglucanase and Caldocellum saccharolyticum endoglucanase B. It is proposed that the enhanced cellulolytic activity of Avicelase I is due to the presence of multiple cellulose-binding sites.

Multiple endo-beta-1,4-glucanase-encoding genes from Bacillus lautus PL236 and characterization of the celB gene

A Bacillus lautus strain was isolated from compost by its ability to degrade microcrystalline Avicel cellulose and acid-swollen cellulose. Three DNA fragments cloned in Escherichia coli encoded at least four endo-beta-1,4-glucanases (EG), of which at least two were contained on one DNA fragment. Another fragment, of 2.5 kb and carrying celB, was cloned in the shuttle-vector plasmid, pJKK3-1, and expressed in E. coli and Bacillus subtilis. The fragment was sequenced and shown to encode a 62-kDa protein, which was found as a 56-kDa mature and active EG in extracts of E. coli and in the supernatant of B. subtilis. The deduced amino acid (aa) sequence has a homology of 37% identical aa on a stretch of 295 aa to EG-E of Clostridium thermocellum. A low level of homology is detected with the Bacillus-type EG.

Nucleotide sequence of the Ruminococcus albus SY3 endoglucanase genes celA and celB

The complete nucleotide sequences of Ruminococcus albus genes celA and celB coding for endoglucanase A (EGA) and endoglucanase B (EGB), respectively, have been determined. The celA structural gene consists of an open reading frame of 1095 bp. Confirmation of the nucleotide sequence was obtained by comparing the predicted amino acid sequence with that derived by N-terminal analysis of purified EGA. The celB structural gene consists of an open reading frame of 1227 bp; 7 bp upstream of the translational start codon of celB is a typical gram-positive Shine-Dalgarno sequence. The deduced N-terminal region of EGB conforms to the general pattern for the signal peptides of secreted prokaryotic proteins. The complete celB gene, cloned into pUC vectors, caused lethality in Escherichia coli. In contrast, celA cloned in pUC18, under the control of lacZp, directed high-level synthesis of EGA in E. coli JM83. EGA in cell-free extract, purified to near homogeneity by ion-exchange chromatography, had a Mr of 44.5 kDa. Gene deletion and subcloning studies with celA revealed that EGA hydrolysed both CMC and xylan, and did not contain discrete functional domains. EGA and EGB showed considerable homology with each other, in addition to exhibiting similarity with Eg1 (R. albus), EGE (Clostridium thermocellum) and End (Butyrivibrio fibrisolvens).

Endoglucanase E, produced at high level in Escherichia coli as a lacZ′ fusion protein, is part of the Clostridium thermocellum cellulosome

The Clostridium thermocellum celE gene was expressed at high level in Escherichia coli TG1 when placed under the transcriptional and translational control of lacZ' in pUC18; in the presence of a multicopy plasmid (pNM52) containing the lacIq gene, expression of full-length and truncated forms of celE was regulated by isopropyl-beta-D-thiogalactopyranoside. Catalytically active endoglucanase E (EGE) produced by E. coli was subject to proteolytic processing. The main protein species produced from full-length and truncated forms of celE was around 40 kDa in size and had an N-terminal amino acid sequence corresponding to that derived for mature EGE from the nucleotide sequence; in addition, larger species of about 75 kDa, presumably corresponding to full-size EGE, were produced by E. coli containing the full-length celE gene. Even after removal of the signal peptide sequence, EGE produced by E. coli was secreted into the periplasm. Up to 157 bp could be deleted from the 5' end of the celE gene without affecting the catalytic activity of EGE produced by E. coli. A polypeptide of Mr 86 kDa, immunoreactive with anti-EGE antiserum, was demonstrated in the high-molecular-weight, cellulose-binding multiprotein aggregate recoverable from C. thermocellum culture supernatant.

Cloning and sequencing of a Bacteroides ruminicola B(1)4 endoglucanase gene

Bacteroides ruminicola B(1)4, a noncellulolytic rumen bacterium, produces an endoglucanase (carboxymethylcellulase [CMCase]) that is excreted into the culture supernatant. Cultures grown on glucose, fructose, maltose, mannose, and cellobiose had high specific activities of CMCase (greater than 3 mmol of reducing sugar per mg of protein per min), but its synthesis was repressed by sucrose. B. rumincola did not grow on either ball-milled or acid-swollen cellulose even though the CMCase could hydrolyze swollen cellulose. The CMCase gene was cloned into Escherichia coli, and its nucleotide sequence contained a single open reading frame coding for a protein of 40,481 daltons. The enzyme was overproduced in E. coli under the control of the tac promoter and purified to homogeneity. The N-terminal sequence, amino acid composition, and molecular weight of the purified enzyme were similar to the values predicted from the open reading frame of the DNA sequence. However, the CMCase present in B. ruminicola was found to have a monomer molecular weight of 88,000 by Western immunoblotting. This discrepancy appeared to have resulted from our having cloned only part of the CMCase gene into E. coli. The amino acid sequence of the CMCase showed homology to sequences of beta-glucanases from Ruminococcus albus and Clostridium thermocellum.

The Glu residue in the conserved Asn-Glu-Pro sequence of two highly divergent endo-beta-1,4-glucanases is essential for enzymatic activity

We initially aligned 28 different cellulase sequences in pairwise fashion and found half of them have the sequence -Asn-Glu-Pro- located in a region flanked by hydrophobic-rich amino acids. Based on lysozyme as a model, the glutamate residue could be essential for enzyme function. We tested this possibility by site-directed mutagenesis of the genes coding Bacillus polymyxa and Bacillus subtilis endo-beta-1,4-glucanases. The genes and amino acid sequences of these two enzymes show very little similarity. Change of Glu-194 and Glu-169 to the isosteric glutamine form in these respective enzymes resulted in a dramatic loss of CMCase activity which could be restored by reverse mutation. Similar mutations to less-conserved residues, Glu-72 and Glu-147, of the B. subtilis enzyme did not cause any loss of activity.

The N-terminal region of an endoglucanase from Pseudomonas fluorescens subspecies cellulosa constitutes a cellulose-binding domain that is distinct from the catalytic centre

The substrate specificity of an endoglucanase (EGB) from Pseudomonas fluorescens subspecies cellulosa was determined. The enzyme was most active against barley beta-glucan, but showed significant activity against amorphous and crystalline cellulose. EGB was purified to homogeneity by affinity chromatography with crystalline cellulose (Avicel). The Mr of the purified enzyme was 50,000, which is in good agreement with the size of EGB deduced from the nucleotide sequence of the celB gene, coding for EGB. The N-terminal region of the mature form of EGB showed strong homology to another endoglucanase and to a xylanase expressed by the same organism; homologous sequences included highly conserved serine-rich regions. Truncated forms of celB, in which the gene sequence encoding the conserved domain had been deleted, directed the synthesis of a functional endoglucanase that did not bind to crystalline cellulose. This indicates that the conserved region of endoglucanases and xylanases expressed by P. fluorescens subsp. cellulosa constitutes a cellulose-binding domain, which is distinct from the active centre. The possible role of this substrate-binding region is discussed.

Nucleotide sequence of the engXCA gene encoding the major endoglucanase of Xanthomonas campestris pv. campestris

The nucleotide sequence of the gene (engXCA) encoding the major extracellular endoglucanase (ENGXCA) of the phytopathogenic bacterium Xanthomonas campestris pv. campestris (X. c. campestris) was determined and compared with the N-terminal amino acid (aa) sequence of the purified enzyme. An open reading frame of 1479 bp encoding 493 aa was identified, of which the N-terminal 25 aa represent a potential signal peptide. Determination of the exact position of a Tn5 insertion within engXCA, which did not reduce the encoded enzyme activity, indicated that the C-terminal region of the protein is not crucial for ENGXCA activity. Comparison of the complete deduced aa sequence with those deduced from other endoglucanase- and exoglucanase-encoding genes revealed a region with a high degree of homology, located towards the C terminus of the protein. These data indicate that the X. c. campestris ENGXCA may have a domain structure similar to that of many other bacterial and fungal cellulolytic enzymes. Hydrophobic cluster analysis was performed on the deduced aa sequence. Comparison of this analysis with those of 30 other cellulase sequences belonging to six different families indicated that the X. c. campestris enzyme can be classified in family A. The two aa residues which had previously been identified as 'potentially catalytic' within this family of cellulases, are conserved in the X. c. campestris ENGXCA.

Nucleotide sequence and deletion analysis of the cellulase-encoding gene celH of Clostridium thermocellum

The complete nucleotide sequence of the celH gene of Clostridium thermocellum was determined. The open reading frame extended over 2.7-kb DNA fragment and encoded a 900-amino acid (aa) protein (Mr 102,301) which hydrolyzes carboxymethylcellulose, p-nitrophenyl-beta-D-cellobioside, methylumbelliferyl- beta-D-cellobioside, barley beta-glucan, and larchwood xylan. The N terminus showed a typical signal peptide, and a cleavage site after Ser44 was predicted. Two Pro-Thr-Ser-rich regions divided the protein into three approximately equal domains. The central 328-aa region was similar to the N-terminal part, carrying the active site, of C. thermocellum endoglucanase E (EGE; 30.2%). The C-terminal region ended with two conserved 24-aa stretches showing close similarity with those previously described in EGA, EGB, EGD, EGE, EGX, and xylanase from C. thermocellum. Deletions of celH removing up to 327 codons from the 5' end and up to 245 codons from the 3' end of the coding sequence did not affect enzyme activity, confirming that the central domain was indeed responsible for catalytic activity. Production of truncated EGH in Escherichia coli was increased up to 120-fold by fusing fragments containing the 3' portion of the gene with the start of lacZ' present in pTZ19R.

Cloning, sequence analysis, and expression of genes encoding xylan-degrading enzymes from the thermophile "Caldocellum saccharolyticum"

A lambda recombinant bacteriophage coding for xylanase and beta-xylosidase activity has been isolated from a genomic library of the extremely thermophilic anaerobe "Caldocellum saccharolyticum." Partial Sau3AI fragments of the lambda recombinant DNA were ligated into pBR322. A recombinant plasmid with an insertion of ca. 7 kilobases of thermophilic DNA expressing both enzymatic activities was isolated. The location of the genes has been established by analyzing deletion derivatives, and the DNA sequence of 6.067 kilobases of the insert has been determined. Five open reading frames (ORFs) were found, one of which (ORF1; Mr 40,455) appears to code for a xylanase (XynA) which also acts on o-nitrophenyl-beta-D-xylopyranoside. Another, ORF5 (Mr 56,365), codes for a beta-xylosidase (XynB). The xynA gene product shows significant homology with the xylanases from the alkalophilic Bacillus sp. strain C125 and Clostridium thermocellum.

Calcium-binding affinity and calcium-enhanced activity of Clostridium thermocellum endoglucanase D

Clostridium thermocellum endoglucanase D (EC 3.2.1.4: EGD), which is encoded by the celD gene, was found to bind Ca2+ with an association constant of 2.03 x 10(6) M-1. Ca2+ stimulated the activity of EGD towards swollen Avicel by 2-fold. In the presence of Ca2+, the Kd of the enzyme towards p-nitrophenyl-beta-D-cellobioside and carboxymethylcellulose was decreased by 4-fold. Furthermore, Ca2+ increased the half-life of the enzyme at 75 degrees C from 13 to 47 min. Since the 3' sequence of celD encodes a duplicated region sharing similarities with the Ca2+-binding site of several Ca2+-binding proteins, a deleted clone was constructed and used to purify a truncated form of the enzyme which no longer contained the duplicated region. The truncated enzyme was very similar to EGD expressed from the intact gene with respect to activity, Ca2(+)-binding kinetics and Ca2+ effects on substrate binding and thermostability. Thus the latter parameters do not appear to be mediated through the duplicated conserved region.

Molecular genetics of glucan metabolism in oral streptococci

Computer analysis of the primary amino acid sequences deduced from nucleotide sequences of cloned genes from Streptococcus mutans and Strep. downei was used to examine features of enzymes involved in formation and degradation of glucans. All 4 glucosyltransferases for which sequence data are available show a common structure characterized by a series of reiterated repeats in the carboxy-terminal one third of the molecule. These repeats are also found in Strep. mutans glucan-binding protein and resemble those found in enzymes from other bacteria with binding properties.

Sequence analysis of the Clostridium cellulolyticum endoglucanase-A-encoding gene, celCCA

The nucleotide sequence of a Clostridium cellulolyticum endo-beta-1,4- glucanase (EGCCA)-encoding gene (celCCA) and its flanking regions, was determined. An open reading frame (ORF) of 1425 bp was found, encoding a protein of 475 amino acids (aa). This ORF began with an ATG start codon and ended with a TAA ochre stop codon. The N-terminal region of the EGCCA protein resembled a typical signal sequence of a Gram-positive bacterial extracellular protein. A putative signal peptidase cleavage site was determined. EGCCA, without a signal peptide, was found to be composed of more than 35% hydrophobic aa and to have an Mr of 50715. Comparison of the encoded sequence with other known cellulase sequences showed the existence of various kinds of aa sequence homologies. First, a strong homology was found between the C-terminal region of EGCCA, containing a reiterated stretch of 24 aa, and the conserved reiterated region previously found to exist in four Clostridium thermocellum endoglucanases and one xylanase from the same organism. This region was suspected of playing a role in organizing the cellulosome complex. Second, an extensive homology was found between EGCCA and the N-terminal region of the large endoglucanase, EGE, from C. thermocellum, which suggests that they may have a common ancestral gene. Third, a region, which extended for 21 aa residues beginning at aa + 127, was found to be homologous with regions of cellulases belonging to Bacilli, Clostridia and Erwinia chrysanthemi.

Recent advances in the genetics of the clostridia

Several laboratories around the world have started work on genetic analysis of clostridia. Interest in this diverse group of anaerobic organisms has grown with increasing awareness of the benefits that may accrue from their biotechnological exploitation. Research to date has focussed on construction of shuttle vectors containing replicons from clostridial and streptococcal plasmids, development of methods for transferring genes, and molecular cloning of genes--especially those involved in toxigenicity, fermentative metabolism and polysaccharide utilization. In selected species gene transfer by protoplast transformation, electroporation and conjugation has been accomplished and transposable elements have been introduced. It can be anticipated that our understanding of the molecular biology of these interesting organisms will grow rapidly in the future, bringing with it improved prospects for rational biotechnological exploitation.

Structure of a Ruminococcus albus endo-1,4-beta-glucanase gene

A chromosomal DNA fragment encoding an endo-1,4-beta-glucanase I (Eg I) gene from Ruminococcus albus cloned and expressed in Escherichia coli with pUC18 was fully sequenced by the dideoxy-chain termination method. The sequence contained a consensus promoter sequence and a structural amino acid sequence. The initial 43 amino acids of the protein were deduced to be a signal sequence, since they are missing in the mature protein (Eg I). High homology was found when the amino acid sequence of the Eg I was compared with that of endoglucanase E from Clostridium thermocellum. Codon usage of the gene was not biased. These results suggested that the properties of the Eg I gene from R. albus was specified from the known beta-glucanase genes of the other organisms.

Cloning and sequencing of an endoglucanase (end1) gene from Butyrivibrio fibrisolvens H17c

The nucleotide sequence of a 2.8 kb DNA segment containing an endoglucanase gene (end1) from Butyrivibrio fibrisolvens H17c was determined. The B. fibrisolvens H17c gene was expressed from its own regulatory region in Escherichia coli and three putative consensus promoter sequences were identified upstream of a ribosome binding site and an ATG start codon. The complete amino acid sequence (547 residues) was deduced and homology with the Clostridium thermocellum celE gene product (EGE) was demonstrated. The endoglucanase contained a typical amino-terminal signal sequence and five repeated sequences (PDPTPVD) between amino acids 412-447. The endoglucanase showed relatively high endoglucanase activity against endoglucanase-specific substrates with beta 1-4 linkages but low activity against xylan and an exoglucanase-specific substrate, p-nitrophenyl-beta-D-cellobioside.

Purification and characterization of endoglucanase C of Cellulomonas fimi, cloning of the gene, and analysis of in vivo transcripts of the gene

Two nonglycosylated endoglucanases which bind to Sephadex were purified from culture supernatants of Cellulomonas fimi grown on microcrystalline cellulose. Their Mrs were 120,000 and 130,000. The N-terminal amino acid sequences of the enzymes were identical, suggesting that the enzymes were related. A DNA fragment encoding this N-terminal sequence was cloned in Escherichia coli. The nucleotide sequence corresponding to the N-terminal amino acid sequence was preceded by a sequence encoding a typical leader peptide. Transcripts hybridizing to the cloned fragment were detected in total RNA isolated from C. fimi cells grown on carboxymethyl cellulose but not from cells grown on glycerol or glucose. Transcription started at a cluster of sites 53 to 59 nucleotides upstream of a GUG translation initiation codon and terminated at either of two closely spaced C residues immediately downstream of a region of potential secondary structure. The size of the transcript was approximately 3.5 kilobases, sufficient to encode a polypeptide of 130 kilodaltons. The 130-kilodalton polypeptide is designated endoglucanase C (CenC), and the gene encoding it is designated cenC.

Structure of the cel-3 gene from Fibrobacter succinogenes S85 and characteristics of the encoded gene product, endoglucanase 3

The cel-3 gene cloned from Fibrobacter succinogenes into Escherichia coli coded for the enzyme EG3, which exhibited both endoglucanase and cellobiosidase activities. The gene had an open reading frame of 1,974 base pairs, coding for a protein of 73.4 kilodaltons (kDa). However, the enzyme purified from the osmotic shock fluid of E. coli was 43 kDa. The amino terminus of the 43-kDa protein matched amino acid residue 266 of the protein coded for by the open reading frame, indicating proteolysis in E. coli. In addition to the 43-kDa protein, Western immunoblotting revealed a 94-kDa membranous form of the enzyme in E. coli and a single protein of 118 kDa in F. succinogenes. Thus, the purified protein appears to be a proteolytic degradation product of a native protein which was 94 kDa in E. coli and 118 kDa in F. succinogenes. The discrepancy between the molecular weight expected on the basis of the DNA sequence and the in vivo form may be due to anomalous migration during electrophoresis, to glycosylation of the native enzyme, or to fatty acyl substitution at the N terminus. One of two putative signal peptide cleavage sites bore a strong resemblance to known lipoprotein leader sequences. The purified 43-kDa peptide exhibited a high Km (53 mg/ml) for carboxymethyl cellulose but a low Km (3 to 4 mg/ml) for lichenan and barley beta-glucan. The enzyme hydrolyzed amorphous cellulose, and cellobiose and cellotriose were the major products of hydrolysis. Cellotriose, but not cellobiose, was cleaved by the enzyme. EG3 exhibited significant amino acid sequence homology with endoglucanase CelC from Clostridium thermocellum, and as with both CelA and CelC of C. thermocellum, it had a putative active site which could be aligned with the active site of hen egg white lysozyme at the highly conserved amino acid residues Asn-44 and Asp-52.

Conserved serine-rich sequences in xylanase and cellulase from Pseudomonas fluorescens subspecies cellulosa: internal signal sequence and unusual protein processing

The complete nucleotide sequence of the xynA gene coding for a xylanase (XYLA) expressed by Pseudomonas fluorescens subspecies cellulosa, has been determined. The structural gene consists of an open reading frame of 1833 bp followed by a TAA stop codon. Confirmation of the nucleotide sequence was obtained by comparing the predicted amino acid sequence with that derived by N-terminal analysis of purified forms of the xylanase. The signal peptide present at the N terminus of mature XYLA closely resembles signal peptides of other secreted proteins. Truncated forms of the xylanase gene, in which the sequence encoding the N-terminal signal peptide had been deleted, still expressed coli. XYLA contains domains which are homologous to an endoglucanase expressed by the same organism. These structures include serine-rich sequences. Bal31 deletions of xynA revealed the extent to which these conserved sequences, in XYLA, were essential for xylanase activity. Downstream of the TAA stop codon is a G + C-rich region of dyad symmetry (delta G = 24 kcal) characteristic of E. coli Rho-independent transcription terminators.

Cellulase families revealed by hydrophobic cluster analysis

The amino acid sequences of 21 beta-glycanases have been compared by hydrophobic cluster analysis. Six families of cellulases have been identified on the basis of primary structure homology: (A) endoglucanases B, C and E of Clostridium thermocellum; endoglucanases of Erwinia chrysanthemi and Bacillus sp.; endoglucanase III of Trichoderma reesei; endoglucanase I of Schizophyllum commune; (B) cellobiohydrolase II of T. reesei; endoglucanases of Cellulomonas fimi and Streptomyces sp; (C) cellobiohydrolases I of T. reesei and of Phanerochaete chrysosporium; endoglucanase I of T. reesei; (D) endoglucanase A of C. thermocellum and an endoglucanase from Ce. uda; (E) endoglucanase D of C. thermocellum and an endoglucanase from Pseudomonas fluorescens; (F) xylanases of C. thermocellum and of Cryptococcus albidus and the cellobio-hydrolase of Ce. fimi. For each family, conserved potentially catalytic residues have have been listed and previous allocations of the active-site residues are evaluated in the light of the alignment of the amino acid sequences. A strong homology is also reported for the putative cellulose-binding domains of cellulases of Ce. fimi and of P. fluorescens.

Identification of three distinct Clostridium thermocellum xylanase genes by molecular cloning

Three genes coding for xylanase synthesis in Clostridium thermocellum were cloned and expressed in Escherichia coli. Genomic DNA from Clostridium thermocellum was digested to completion with HindIII, BamHI, and SalI. The fragments were ligated into the corresponding sites of pUC19 and transformed into Escherichia coli. Two of the genes encoded for xylanases which depolymerized xylans but were unable to extensively convert these substrates to reducing sugar. The third gene encoded for an enzyme that extensively hydrolyzed xylan. The insert containing the latter gene was subjected to extensive mapping and was found to encode for a xylanase with a molecular weight of approximately 25,000. The protein product of the cloned gene was obtained in a relatively pure form by heat treatment, ion exchange and gel permeation steps. The enzyme was quite stable to high temperatures with a half-life of 24 h at 70 degrees C.

Animal biotechnology

Biotechnology has taken two directions in efforts to speed up animal production above the rates achievable by selective breeding. Recombinant DNA methods have been used to engineer protein gene products for direct administration to livestock, as in recombinant growth hormone to stimulate lactation in dairy cows or yield faster-growing, leaner carcasses in meat animals. Cloned cellulolytic genes have been inserted into ruminal microorganisms with a view to improving ruminant nutrition. The other direction is to use advanced breeding technologies to enhance performance. These include laboratory culture of large numbers of viable embryos for non-surgical transfer to surrogate mothers, development of methods for sexing sperm and embryos, cloning embryos by nuclear transplantation and gene transfer to create livestock with superior performance traits. In all cases material progress will depend upon a deeper understanding of the underlying physiological and developmental control mechanisms and public confidence that due regard is being paid to animal welfare, and to social and environmental implications.

Expression of a Clostridium thermocellum endoglucanase gene in Lactobacillus plantarum

Recombinant plasmid pM25 containing the celE gene of Clostridium thermocellum, which codes for an enzymatically active endoglucanase, was transformed into Lactobacillus plantarum by electroporation. Strains harboring pM25 expressed thermostable endoglucanase, which was found predominantly in the culture medium. Two other plasmids, pGK12 and pSA3, were transformed into L. plantarum, and the stability of each plasmid was evaluated.

Nucleotide sequence of the Clostridium thermocellum bgIB gene encoding thermostable beta-glucosidase B: homology to fungal beta-glucosidases

The nucleotide sequence of the bglB gene, coding for the thermostable beta-glucosidase B of Clostridium thermocellum was determined. The coding region of 2265 bp was identified by comparison with the N-terminal amino acid sequence of beta-glucosidase B purified from Escherichia coli. The derived amino acid sequence corresponding to a polypeptide of Mr 84,100 was confirmed by sequencing of the C-terminal peptide generated by cleavage with cyanogen bromide. The protein bears no resemblance to other bacterial beta-glucosidase sequences. However, extensive regions of homology were identified between the C. thermocellum enzyme and fungal beta-glucosidases. The N-terminal homologous region contains an amino acid sequence very similar to the active site of beta-glucosidase A3 from Aspergillus wentii. The striking sequence similarities between C. thermocellum beta-glucosidase B and Kluyveromyces fragilis beta-glucosidase suggest the possibility of a genetic exchange between thermophilic anaerobic bacteria and yeasts.