Rumen bacterium Pseudobutyrivibrio xylanivorans Mz5(T) produces multiple xylanases that account for its high xylanolytic activity. The smallest xylanase (XynT) accounts for the majority of the activity in the later growth phases. This enzyme was purified to homogeneity by ammonium sulfate precipitation and hydrophobic interaction chromatography and had a MW of 30 kDa. Electrophoresis under native conditions and isoelectric focusing showed microheterogeneity of XynT, with two pls: 5.1 and 5.9. This could be due to different post-translational processing, partial proteolysis or aggregation. Nevertheless, the single N-terminal sequence revealed the relatedness of XynT to xylanases from family 11 of glycosyl hydrolases. XynT is most active in the physiological conditions found in bovine rumen (38 degreesC, pH 5.6) and acts as an endoxylanase that releases xylooligosaccharides from xylan. It is inactive towards other polysaccharides. XynT could be used as a feed additive for animals in order to diminish health problems and enhance proliferation of beneficial microflora. (C) 2003 Elsevier Inc. All rights reserved.