| Summary: | Most common industrial xylanases are produced from filamentous fungi. In this study, the codon-optimized <i>xynA</i> gene encoding xylanase A from the fungus <i>Penicilium citrinum</i> was successfully synthesized and expressed in the yeast <i>Pichia pastoris</i>. The levels of secreted enzyme activity under the control of glyceraldehyde-3-phosphate dehydrogenase (<i>P<sub>GAP</sub></i>) and alcohol oxidase 1 (<i>P<sub>AOX1</sub></i>) promoters were compared. The Pc Xyn11A was produced as a soluble protein and the total xylanase activity under the control of <i>P<sub>GAP</sub></i> and <i>P<sub>AOX1</sub></i> was 34- and 193-fold, respectively, higher than that produced by the native strain of <i>P. citrinum.</i> The Pc Xyn11A produced under the control of the <i>P<sub>AOX1</sub></i> reached a maximum activity of 676 U/mL when induced with 1% (<i>v/v</i>) methanol every 24 h for 5 days. The xylanase was purified by ion exchange chromatography and then characterized. The enzyme was optimally active at 55 °C and pH 5.0 but stable over a broad pH range (3.0−9.0), retaining more than 80% of the original activity after 24 h or after pre-incubation at 40 °C for 1 h. With birchwood xylan as a substrate, Pc Xyn11A showed a <i>K</i><sub>m(app)</sub> of 2.8 mg/mL, and a <i>k</i><sub>cat</sub> of 243 s<sup>−1</sup>. The high level of secretion of Pc Xyn11A and its stability over a wide range of pH and moderate temperatures could make it useful for a variety of biotechnological applications.
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