Molecular cloning and expression in Escherichia coli of a cellodextrinase gene from Bacteroides succinogenes S85

Biochemical characterization of MI-ENG1, a family 5 endoglucanase secreted by the root-knot nematode Meloidogyne incognita

A beta-1,4-endoglucanase named MI-ENG1, homologous to the family 5 glycoside hydrolases, was previously isolated from the plant parasitic root-knot nematode Meloidogyne incognita. We describe here the detection of the enzyme in the nematode homogenate and secretion and its complete biochemical characterization. This study is the first comparison of the enzymatic properties of an animal glycoside hydrolase with plant and microbial enzymes. MI-ENG1 shares many enzymatic properties with known endoglucanases from plants, free-living or rumen-associated microorganisms and phytopathogens. In spite of the presence of a cellulose-binding domain at the C-terminus, the ability of MI-ENG1 to bind cellulose could not be demonstrated, whatever the experimental conditions used. The biochemical characterization of the enzyme is a first step towards the understanding of the molecular events taking place during the plant-nematode interaction.

Two beta-glucosidase activities in Fibrobacter succinogenes S85

Few bacteria are capable of degrading crystalline cellulose but there is considerable interest in the properties of enzyme systems with this capability. In the bovine and ovine rumen the principal cellulolytic bacterium is Fibrobacter (formerly Bacteroides) succinogenes. The cellulase system of this organism is composed of multiple enzyme components, including a constitutive and cell-associated beta-glucosidase active against cellobiose. The properties of the beta-glucosidase activity have been investigated with the chromogenic substrate p-nitrophenyl beta-D-glucoside (pNPG). Hydrolytic activity against pNPG was located primarily in the cytoplasm and the cytoplasmic membrane but showed a gradual migration to the periplasm during growth on either glucose or cellobiose. Activity against cellobiose was found in the periplasm in significant amounts in all growth phases. Of the beta-glucosides tested, only cellobiose and pNPG were hydrolysed by crude cell extracts. In the presence of cellobiose, however, the rate of hydrolysis of pNPG was stimulated up to 10-fold, and extracts hydrolysed methylumbelliferyl beta-D-glucoside, 5-bromo-4-chloro-3-indolyl beta-D-glucoside, arbutin and aesculin. Activities against pNPG in the presence and absence of cellobiose displayed similar instability in the presence of oxygen; both were stabilized by dithiothreitol and the temperature and pH optima were identical. A significant proportion of the membrane-associated beta-glucosidase was released by treatment with 0.3 mol/1 KCl, and fractionation by chromatography on CM-cellulose showed the presence of two activities against pNPG, only one of which was stimulated by cellobiose.

Type II DNA restriction-modification system and an endonuclease from the ruminal bacterium Fibrobacter succinogenes S85

Fibrobacter succinogenes is an important cellulolytic bacterium found in the rumen and cecum of herbivores. Numerous attempts to introduce foreign DNA into F. succinogenes S85 have failed, suggesting the presence of genetic barriers in this organism. Results from this study clearly demonstrate that F. succinogenes S85 possesses a type II restriction endonuclease, FsuI, which recognizes the sequence 5'-GG(A/T)CC-3'. Analysis of the restriction products on sequencing gels showed that FsuI cleaves between the two deoxyguanosine residues, yielding a 3-base 5' protruding end. These data demonstrate that FsuI is an isoschizomer of AvaII. A methyltransferase activity has been identified in the cell extract of F. succinogenes S85. This activity modified DNA in vitro and protected the DNA from the restriction by FsuI and AvaII. DNA modified in vivo by a cloned methylase gene, which codes for M.Eco47II, also protected the DNA from restriction by FsuI, suggesting that FsuI is inhibited by methylation at one or both deoxycytosine residues of the recognition sequence. The methyltransferase activity in F. succinogenes S85 is likely modifying the same deoxycytosine residues, but the exact site(s) is unknown. A highly active DNase (DNase A) was also isolated from the cell extract of this organism. DNase A is an endonuclease which showed high activity on all forms of DNA (single stranded, double-stranded, linear, and circular) but no activity on RNA. In vitro, the DNase A hydrolyzed F. succinogenes S85 DNA extensively, indicating the lack of protection against hydrolysis by this enzyme. In the presence of Mg2+, DNA was hydrolyzed to fragments of 8 to 10 nucleotides in length. The presence of DNase A and the type II restriction-modification system of F. succinogenes S85 may be the barriers preventing the introduction of foreign DNA into this bacterium.

The hydrolysis of lucerne cell-wall monosaccharide components by monocultures or pair combinations of defined ruminal bacteria

The defined ruminal bacterial strains Fibrobacter succinogenes S85, Ruminococcus flavefaciens FD1, Ruminococcus albus 7, Butyrivibrio fibrisolvens D1, and Bacteroides ruminicola GA33 were grown, in monocultures or as combinations of pair strains, on isolated lucerne cell-walls (CW) as the sole carbohydrate substrate. Fibrobacter succinogenes S85 was the dominant strain determining extent of CW hydrolysis in all combinations with S85. The hydrolysis of cellulose, xylan, hemicellulose side-sugars, and total CW monosaccharides by pure S85 were: 58.8, 47.3, 66.9 and 57.0%, respectively. The strains combination S85 plus D1 comprised the highest complementary effect, increasing significantly the hydrolysis of cellulose and total CW monosaccharides by 16% and 13%, respectively, above the values obtained by pure S85. This complementation was expressed also in growth pattern of bacteria. The monocultures of FD1, D1 and GA33 had very little hydrolytic effect on lucerne cellulose, but higher effects on xylan and hemicellulose side-sugars. The combinations D1 plus GA33 and 7 plus GA33 were complementary in the hydrolysis of all CW polysaccharides. The combinations FD1 plus D1, FD1 plus GA33, and 7 plus D1 were complementary only with respect to hemicellulose hydrolysis. On the other hand, the cellulolytic combinations S85 plus FD1, S85 plus 7 and FD1 plus 7 demonstrated negative interactions in lucerne CW polysaccharides hydrolysis. Under scanning electron microscopy (SEM), S85 comprised the most dense layer of bacterial cell mass attached to and colonized on CW particles. The cell surface topology of the cellulolytic strains S85, FD1 and 7 attached to CW particles was specified by a coat of characteristic protuberant structures.

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.

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.

DNA sequence of a Fibrobacter succinogenes mixed-linkage beta-glucanase (1,3-1,4-beta-D-glucan 4-glucanohydrolase) gene

The DNA sequence of a mixed-linkage beta-glucanase (1,3-1,4-beta-D-glucan 4-glucanohydrolase [EC 3.2.1.73]) gene from Fibrobacter succinogenes cloned in Escherichia coli was determined. The general features of this gene are very similar to the consensus features for other gram-negative bacterial genes. The gene product was processed for export in E. coli. There is a high level of sequence homology between the structure of this glucanase and the structure of a mixed-linkage beta-glucanase from Bacillus subtilis. The nonhomologous region of the amino acid sequence includes a serine-rich region containing five repeats of the sequence Pro-Xxx-Ser-Ser-Ser-Ser-(Ala or Val) which may be functionally related to the serine-rich region observed in Pseudomonas fluorescens cellulase and the serine- and/or threonine-rich regions observed in Cellulomonas fimi endoglucanase and exoglucanase, in Clostridium thermocellum endoglucanases A and B, and in Trichoderma reesei cellobiohydrolase I, cellobiohydrolase II, and endoglucanase I.

Nucleotide sequence of the celA gene encoding a cellodextrinase of Ruminococcus flavefaciens FD-1

The nucleotide sequence of a 3.6 kb DNA fragment containing a cellodextrinase gene (celA) from Ruminococcus flavefaciens FD-1 was determined. The gene was expressed from its own regulatory region in Escherichia coli and a putative consensus promoter sequence was identified upstream of a ribosome binding site and a TTG start codon. The complete amino acid sequence of the CelA enzyme (352 residues) was deduced and showed no significant homology to cellulases from other organisms. Two lysozyme-type active sites were found in the amino-terminal third of the enzyme. In E. coli the cloned Cel A protein was translocated into the periplasm. The lack of a typical signal sequence, and the results of transposon phoA mutagenesis experiments indicated that CelA is secreted by a mechanism other than a leader peptide.

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.

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.