Identification and characterization of the first β-1,3-d-xylosidase from a gram-positive bacterium, Streptomyces sp. SWU10.

In previous reports, we characterized four endo-xylanases produced by Streptomyces sp. strain SWU10 that degrade xylans to several xylooligosaccharides. To obtain a set of enzymes to achieve complete xylan degradation, a β-d-xylosidase gene was cloned and expressed in Escherichia coli, and the recombinant protein, named rSWU43A, was characterized. SWU43A is composed of 522 amino acids and does not contain a signal peptide, indicating that the enzyme is an intracellular protein. SWU43A was revealed to contain a Glyco_hydro_43 domain and possess the three conserved amino acid residues of the glycoside hydrolase family 43 proteins. The molecular mass of rSWU43A purified by Ni-affinity column chromatography was estimated to be 60kDa. The optimum reaction conditions of rSWU43A were pH 6.5 and 40°C. The enzyme was stable up to 40°C over a wide pH range (3.1-8.9). rSWU43A activity was enhanced by Fe2+ and Mn2+ and inhibited by various metals (Ag+ , Cd2+ , Co2+ , Cu2+ , Hg2+ , Ni2+ , and Zn2+ ), d-xylose, and l-arabinose. rSWU43A showed activity on p-nitrophenyl-β-d-xylopyranoside and p-nitrophenyl-α-l-arabinofuranoside substrates, with specific activities of 0.09 and 0.06U/mg, respectively, but not on any xylosidic or arabinosidic polymers. rSWU43A efficiently degraded β-1,3-xylooligosaccharides to produce xylose but showed little activity towards β-1,4-xylobiose, with specific activities of 1.33 and 0.003U/mg, respectively. These results demonstrate that SWU43A is a β-1,3-d-xylosidase (EC 3.2.1.72), which to date has only been described in the marine bacterium Vibrio sp. Therefore, rSWU43A of Streptomyces sp. is the first β-1,3-xylosidase found in gram-positive bacteria. SWU43A could be useful as a specific tool for the structural elucidation and production of xylose from β-1,3-xylan in seaweed cell walls.