TECHNOLOGY FOCUS - BIOTECHNOLOGY - Preferred Polypeptide: (I) has more than 80%, preferably 90% of sequence identity with (S1). (I) has an optimum enzymatic activity at a pH between 3.5 and 4.5, and at a temperature between 45 and 60degreesC.Preferred Nucleotide Sequence: (II) has more than 80%, preferably more than 90% of sequence identity with (S2).Preferred Vector: (IV) is a plasmid incorporated in Escherichia coli and having the deposit number MUCL-43962.Preferred Host Cell: (V) is chosen from bacteria, fungi and yeast, and preferably Pichia pastoris MUCL-43962 strain. EXAMPLE - Wild type strain X-33 of Pichia pastoris was transformed by electroporation with the pSA4 plasmid. The pSA4 plasmid contained XYLSA1 coding sequence in fusion with the yeast invertase signal sequence and under the control of AOX1 promoter. The transformants were selected on the YPD plates with zeocin. Integration was tested by PCR on 10 transformants, all were positive. Four transformants (named pSA4.1, pSA4.2, pSA4.3, pSA4.4) were screened for xylanase activity in small-culture analysis. They were grown under non-induced conditions in 25 ml of BMGY medium (1% yeast extract, 2% peptone, 100 mM potassium phosphate, pH 6, 1.34% YNB, biotin, 1% glycerol) for 16 hours at 30degreesC. Cells were harvested by centrifuging at 3000 g for 5 minutes. Cell pellet was suspended to an optical density at 600 nm equal to 1 in methanol medium BMMY (1% yeast extract, 2% peptone, 100 mM potassium phosphate, pH 6, 1.34% YNB, biotin, 0.5% methanol, induced conditions). Culture was returned to incubator to continue growth. Aliquots of the culture supernatant (500 mul) obtained at different times (5, 21, 28, 45, 54, 60 hours) of induction were subjected xylanase activity assay and to sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The best clone (PSA 4.1) on basis of activity was used to inoculate a 5 l fermentator and fermentation was carried out. The enzyme with xylanolytic activity expressed in P.pastoris was secreted and analyzed on SDS-PAGE gel. The protein was purified in order to separate from it from the trace of alpha-amylase present in the culture supernatants of the transformants. The pH and temperature dependent of the activity for xylanolytic enzyme secreted by one P.pastoris transformant was analyzed. The activity was measured in a Mcllvain's buffer (0.2 M sodium phosphate, 0.1 M citric acid, pH 3, 4) at various temperature. The maximum activity was observed around 50degreesC and at the temperature, the optimum pH was about 4. Delignification of kraft pulp by xylanase was evaluated. Hardwood kraft pulp at 9% was incubated 2 hours at 60degreesC in 50 mM acetate buffer (pH 3) with 100 UI of xylanase/g of the dry pulp. During incubation, liberation of lignin was followed by absorbance at 280 nm. The results showed that the xylanase had the ability to delignify hardwood kraft pulp.