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The molecular cloning and characterization of a Beta-glucosidase gene from an Agrobacterium Wakarchuk, Warren William


The β-glucosidase (Abg) from ATCC 21400, an Agrobacterium species, was purified to homogeneity. The protein was cleaved with cyanogen bromide and the peptides were purified by reversed phase high pressure liquid chromatography. The partial amino-acid sequences for three CNBr peptides, CNBr1, CNBr2 and CNBr3, were determined by automated Edman degradation. A sequence from CNBr2 was used to synthesize a mixture of oligonucleotides which was used as a hybridization probe to identify a recombinant DNA clone carrying the gene for β-glucosidase. A single clone was isolated which expressed an enzymatic activity that hydrolyzed several β-glucosides. The enzymatic activity produced by this clone could be adsorbed by rabbit antiserum raised against the Agrobacterium enzyme. The direction of transcription of the β-glucosidase gene was determined by verifying the DNA sequence 3' to the oligonucleotide probe binding site. After subcloning the gene a high level of expression was obtained in the plasmid vector pUC18 using the lacZ gene promoter. The nucleotide sequence of the 1599 bp insert in pABG5 was determined using the chain terminator method. The start of the protein coding region was determined by aligning the amino terminal sequence of the protein with the predicted amino acid sequence of the cloned gene. The open reading frame was 1387 nucleotides and contained 458 codons. The molecular weight calculated from the deduced amino acid sequence agreed with that observed from both the native and recombinant enzymes. The predicted amino acid composition from the open reading frame matched with that determined for the native β-glucosidase. The stop codon of this coding region was followed by a potential stem loop structure which may be the transcriptional terminator. There was a region of the deduced Abg sequence which had homology to a region from two other β-glucosidase sequences. This region of homology contained a putative active site by analogy with the active site of hen egg white lysozyme.

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