The gene encoding xylanase C from sp. BP-23 was cloned and expressed in The nucleotide sequence of a 3538 bp DNA fragment containing gene was determined, revealing an open reading frame of 3258 bp that encodes a protein of 120567 Da. A comparison of the deduced amino acid sequence of xylanase C with known β-glycanase sequences showed that the encoded enzyme is a modular protein containing three different domains. The central region of the enzyme is the catalytic domain, which shows high homology to family 10 xylanases. A domain homologous to family IX cellulose-binding domains is located in the C-terminal region of xylanase C, whilst the N-terminal region of the enzyme shows homology to thermostabilizing domains found in several thermophilic enzymes. Xylanase C showed an activity profile similar to that of enzymes from mesophilic microorganisms. Maximum activity was found at 45°C, and the enzyme was only stable at 55°C or lower temperatures. Xylotetraose, xylotriose, xylobiose and xylose were the main products from birchwood xylan hydrolysis, whilst the enzyme showed increasing activity on xylo-oligosaccharides of increasing length, indicating that the cloned enzyme is an endoxylanase. A deletion derivative of xylanase C, lacking the region homologous to thermostabilizing domains, was constructed. The truncated enzyme showed a lower optimum temperature for activity than the full-length enzyme, 35°C instead of 45°C, and a reduced thermal stability that resulted in a complete inactivation of the enzyme after 2 h incubation at 55°C.


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