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Journal of Bacteriology, January 2002, p. 600-604, Vol. 184, No. 2
0021-9193/01/$04.00+0 DOI: 10.1128/JB.184.2.600-604.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
-Galactosidase Aga27A, an Enzymatic Component of the Clostridium josui Cellulosome
Faculty of Bioresources, Mie University, Tsu 514-8507, Japan,1 Nagoya Seiraku Co. Ltd., Nagoya 468-8588, Japan2
Received 2 July 2001/ Accepted 17 October 2001
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-galactosidase Aga27A, which comprises a catalytic domain of family 27 of glycoside hydrolases and a dockerin domain responsible for cellulosome assembly. The catalytic domain is highly homologous to those of various
-galactosidases of family 27 of glycoside hydrolases from eukaryotic organisms, especially plants. The recombinant Aga27A
-galactosidase devoid of the dockerin domain preferred highly polymeric galactomannan as a substrate to small saccharides such as melibiose and raffinose. |
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-Galactosidase (EC 3.2.1.22) hydrolyzes
-galactosidic linkages at nonreducing ends in galactose-containing oligosaccharides, galactolipids, and galactomannan (4). Since galactomannan is a hemicellulosic material, the presence of
-galactosidase(s) in the cellulosomes should contribute to the degradation of plant cell walls.
The cipA gene, encoding the scaffolding protein, and some cellulase genes, namely, celB, celD, and celE, were identified in a gene cluster in the C. josui chromosomal DNA (10, 14). In addition, the celA (11) and xynA (7) genes were cloned and characterized along with their translated products. Since we expected that genes encoding cellulases and hemicellulases were clustered in C. josui, we sequenced wide areas around celA and cipA. In this process, we found the aga27A gene, encoding
-galactosidase and classified in family 27 of glycoside hydrolases (reference 13 and information found at the CNRS [Marseille, France] website [http://afmb.cnrs-mrs.fr/CAZY/]), upstream of celA. In this paper, we describe the nucleotide sequence of aga27A and enzyme properties of the recombinant Aga27A
-galactosidase.
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CjCel-12, but the direction of transcription of aga27A was opposite to that of celA (Fig. 1). A 3.5-kb EcoRI fragment of
CjCel-12 containing aga27A was subcloned into the EcoRI site of pBluescript II SK(-), yielding pCj-Aga27A (Fig. 1). The nucleotide sequence of aga27A was determined using an ABI PRISM 310 DNA sequencer system (Perkin-Elmer Applied Biosystems) with a BigDye Terminator sequencing kit (Perkin-Elmer Applied Biosystems) and a series of the subclones. Homology searches in the DDBJ database were carried out with the BLAST program (1).
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FIG. 1. Restriction maps of CjCel-12 and pCj-AgaA (A) and domains of AgaA protein and its derivatives (B). (A) Thin lines correspond to the vector GEM-12 or pBluescript II SK(-), and open bars correspond to cloned DNA fragments. The coding region is represented by solid bars. Arrows indicate the direction of the transcription of agaA and celA. E, EcoRI; H, HindIII; S, Sau3AI; X, XhoI.
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-galactosidase catalytic domains in family 27 from various organisms, especially from plants, e.g., Coffea arabica (coffee; 55% sequence identity) (29), Glycine max (soybean; 54%) (DDBJ/EMBL/GenBank accession no. U12926), Phaseolus vulgaris (kidney bean; 54%) (U12927), Cyamopsis tetragonoloba (guar; 54%) (20), Senna occidentalis (senna; 54%) (A63585), Pseudomonas fluorescens subsp. cellulosa (51%) (12), Homo sapiens (human; 45%) (15), Mortierella vinacea (43%) (22, 23), Mus musculus (mouse; 42%) (19), and Saccharomyces carlsbergensis (40%) (26).
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FIG. 2. Alignment of family 27 catalytic domains of -galactosidases from C. josui (Cj), Caffea arabica (Ca), Cyamopsis tetragonoloba (Ct), G. max (Gm), H. sapiens (Hs), Mus musculus (Mm), Mortierella vinacea (Mv), P. fluorescens subsp. cellulosa (Pf), and Phaseolus vulgaris (Pv). MvI and MvII are -galactosidase I and -galactosidase II, respectively, from Mortierella vinacea. Amino acids which are conserved in at least 7 of the 10 sequences are shaded. Dashes indicate gaps left to improve alignment. Numbers refer to amino acid residues at the beginnings of the respective lines; all sequences are numbered from Met-1 of the peptide.
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-Galactosidases are substantially classified into three groups, families 4, 27, and 36 of glycosyl hydrolases, on the basis of amino acid sequence homology (reference 13 and information found at the CNRS [Marseille, France] website [http://afmb.cnrs-mrs.fr/CAZY/]). Almost all of the
-galactosidases from eukaryotic organisms belong to family 27, although an
-galactosidase of Trichoderma reesei (accession no. Z69254) is a member of family 36. On the other hand, most prokaryotic
-galactosidases have been classified into family 4 or 36. Therefore, the C. josui Aga27A
-galactosidase is, along with P. fluorescens subsp. cellulosa Aga27A
-galactosidase (12), the rare exception. A dockerin domain was found at the C terminus of Aga27A (Fig. 1). Dockerin domains which consist of two duplicated sequences, each of about 22 amino acid residues, are conserved in the components of cellulosomes from C. cellulolyticum, C. cellulovorans, C. josui, and C. thermocellum. Pagès et al. (21) showed that cohesin-dockerin interactions in the C. cellulolyticum and C. thermocellum cellulosomes are species-specific phenomena, and they predicted that four amino acid residues which comprise a repeated pair, AL or AI, in the C. cellulolyticum enzymes and ST in the C. thermocellum enzymes are critical to binding specificity as the recognition code. Recently, Mechaly et al. (17) reported the significance of these residues in the cohesin-dockerin interaction. The dockerin domains of C. josui CelB and CelD were shown to be similar to those of the C. cellulolyticum cellulases; i.e., an AL or AI motif is conserved in the dockerins from C. josui cellulases. A pair of AL motifs was found in the dockerin domain of Aga27A. The presence of a dockerin domain in Aga27A suggested that it is a component of the C. josui cellulosome.
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FIG. 3. Expression of AgaA in C. josui and E. coli. The gel was stained with Coomassie brilliant blue (A) or stained for -galactosidase activity (C). AgaA proteins were detected with a polyclonal mouse antiserum raised against truncated AgaA by Western blot analysis (B). Lane 1, protein mass standards; lane 2, truncated AgaA purified from a recombinant E. coli strain (1 µg); lanes 3, truncated AgaA purified from a recombinant E. coli strain (0.1 µg); lanes 4, cellulosomal proteins of C. josui.
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-Galactosidase activity was measured after a 3-min incubation at 45°C in McIlvaine buffer (a mixture of 0.2 M Na2HPO4 and 0.1 M citric acid [pH 5.5]) in the presence of 10 mM p-nitrophenyl-
-d-galactopyranoside (PNP-Gal; Sigma). One unit of activity was defined as the amount of enzyme releasing 1 µmol of p-nitrophenol from PNP-Gal per min. When melibiose, raffinose, and guar gum (Sigma) were used as the substrates, reducing sugars released from the substrates were measured with the 3,5-dinitrosalicylic acid reagent as described by Miller (18). One unit of activity was defined as the amount of enzyme releasing 1 µmol of galactose from the substrates per min. From Lineweaver-Burk plots, the Km and Vmax values of the enzyme for PNP-Gal were estimated to be 0.81 mM and 92.9 µmol/min/mg, respectively. The optimum pH for activity on PNP-Gal was found to be pH 5.5, and the enzyme was stable in the range of pH 3.0 to 7.0 when it was incubated in buffers of different pHs without the substrate at 30°C for 3 h. The optimum temperature for activity on PNP-Gal was found to be 58°C at pH 5.5. The enzyme was stable at 40°C for 10 min at pH 5.5 in the absence of the substrate. The enzyme displayed similar optimal activities and stabilities for raffinose and guar gum. The truncated Aga27A hydrolyzed the
-1,6-galactoside linkage in guar gum as well as in melibiose and raffinose. The specific activities of the enzyme were 8.3 U/mg for guar gum and 1.7 U/mg for raffinose. The value for guar gum is comparable with that for a T. reesei enzyme, although Aga27A appears to be less active toward raffinose than the fungal enzyme is; i.e., the specific activities of a T. reesei
-galactosidase were 15.3 and 15.2 U/mg for locust bean gum and raffinose, respectively (28). The Km values for Aga27A were determined to be 2.2 and 21.0 mg/ml for guar gum and raffinose, respectively. The release of galactose from guar gum, raffinose, melibiose, and lactose was analyzed by thin-layer chromatography with 1-propanol-nitromethane-water (5:2:3, vol/vol/vol) as the development solvent. The sugars on the plate were visualized with sulfuric acid by heating at 140°C for 5 min. Galactose was detected in the hydrolysates of guar gum, raffinose, and melibiose but not lactose (Fig. 4), showing that the activity of Aga27A is specific for the
-1,6 linkage.
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FIG. 4. Thin-layer chromatography of hydrolysis products from guar gum (A), raffinose (B), and melibiose (C). The reaction mixture was composed of 80 µl of 1% substrate, 80 µl of McIlvaine buffer (pH 5.5), and 40 µl (0.8 U) of enzyme solution. The reaction was done at 35°C, and 20 µl of the reaction mixture was withdrawn at each time indicated. Four microliters of the mixture was used for thin-layer chromatography. Guar, guar gum; Mel, melibiose; Raf, raffinose; Gal, galactose; Glc, glucose; Suc, sucrose; Man, mannanase.
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-Galactosidases can be classified into two groups based on their substrate specificities (5); i.e., one group is specific for small saccharides such as PNP-Gal, melibiose, and raffinose, and the other group can liberate galactose from highly polymerized galactomannans such as guar gum in addition to the small substrates. Some enzymes in family 27 are known to be more active on guar gum than on the small substrates (5). C. josui Aga27A also prefers guar gum to the small substrates. By contrast, bacterial
-galactosidases in family 4 or 36 show high activities toward the small substrates but negligible activities toward guar gum (8, 9). Although P. fluorescens subsp. cellulosa Aga27A, classified in family 27, was capable of hydrolyzing carob galactomannan, its enzyme activity toward this substrate was quite weak, i.e., 0.019 U/mg (12). |
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-d-galactopyranoside as the substrate identified an
-galactosidase with a molecular weight of about 52,000 (Fig. 3C). These results indicate that Aga27A is one of the components of the C. josui cellulosome. Aga27A was detected in the cellulosome fraction prepared from the culture supernatant of C. josui grown on BMC. Although this bacterium grew quite slowly on guar gum as the carbon source, Western blot analysis indicated that the production of Aga27A was not enhanced by the addition of guar gum (data not shown). These observations suggest that the expression of the aga27A gene is not induced by guar gum but is related to cellulose degradation. It is likely that Aga27A contributes to the degradation of galactomannan present in plant cell walls, allowing the cellulosome access to cellulose chains that are buried in galactomannan and are not accessible unless galactomannan is hydrolyzed and removed.
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We thank Yoshikazu Ishida of Toyobo Co., Ltd. for the gift of pCBD-C.
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