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Characterization and expression of Cellulomonas fimi endoglucanase B gene and properties of the gene product from Escherichia coli Owolabi, Joshua Babatunde


In Cellulomonas fimi the cenB gene encodes a secreted endoglucanase (EngB) involved in the degradation of cellulose. The cenB gene carried on a 5.6 kb C fimi DNA fragment encodes a polypeptide of Mr 110,000 in Escherichia coli. The level of expression of the gene was significantly increased by replacing its normal transcriptional and translational regulatory signals with those of the E. coli lac operon. The intact EngB polypeptide is not required for enzymatic activity: active polypeptides of Mr 95,000 and 82,000 also appear in E. coli and a deletion mutant of cenB encodes an active polypeptide of Mr 72,000. The intact and truncated EngB both bind to microcrystalline cellulose. A simple, rapid affinity chromatography procedure on Avicel was developed for the purification of intact EngB and of the 72,000 deletion derivative. Alignment of the amino-terminal amino acid sequence of the purified intact EngB from E. coli with the partial nucleotide sequence of the cloned C. fimi DNA showed that the mature EngB is preceded by a sequence encoding a putative signal polypeptide of 32 amino acids, a translational initiation codon and a sequence resembling an E. coli ribosome binding site 4 nucleotides before the initiation codon. The signal peptide functions and is correctly processed in E. coli, even when its first 15 amino acids are replaced by the first 7 amino acids of β-galactosidase. The truncation of EngB does not affect its export to the periplasm of E.coli. In the intact EngB, 25% of the residues are hydroxyamino acids. It displays features common to endo-β-1 ,4-glucanases, since it has a high activity on carboxymethylcellulose. The kinetic parameters for carboxymethylcellulose hydrolysis of both intact and truncated EngB are not significantly different. C. fimi protease cleaves intact EngB, in a specific manner, to generate two polypeptides of Mr 65,000 and 43,000; the former has the capacity to bind Avicel. A polyclonal antibody raised against the purified intact EngB recognizes a C. fimi extracellular protein of M 110,000 as well as 5 polypeptides of lower molecular weight.

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