The cellulolytic myxobacterium Sorangium cellulosum is able to efficiently degrade many kinds of polysaccharides, but none of the enzymes involved have been characterized. In this paper, a xylanase gene (xynA) was cloned from S. cellulosum So9733-1 using thermal asymmetric interlaced PCR. The gene is composed of 1,209 bp and has only 52.27% G + C content, which is much lower than that of most myxobacterial DNA reported (67-72%). Gene xynA encodes a 402 amino acid protein that contains a single catalytic domain belonging to the glycoside hydrolase family 10. The novel xylanase gene, xynA, was expressed in Escherichia coli BL21 (DE3) and the recombinant protein (r-XynA) was purified by Ni-affinity chromatography. The r-XynA had the optimum temperature of 30-35A degrees C and exhibited 33.3% activity at 5A degrees C and 13.7% activity at 0A degrees C. Approximately 80% activity was lost after 20-min pre-incubation at 50A degrees C. These results indicate that r-XynA is a cold-active xylanase with low thermostability. At 30A degrees C, the K (m) values of r-XynA on beechwood xylan, birchwood xylan, and oat spelt xylan were 25.77 A +/- 4.16, 26.52 A +/- 4.78, and 38.13 A +/- 5.35 mg/mL, respectively. The purified r-XynA displayed optimum activity at pH 7.0. The activity of r-XynA was enhanced by the presence of Ca2+. The r-XynA hydrolyzed beechwood xylan, birchwood xylan, and xylooligosaccharides (xylotriose, xylotetraose, and xylopentose) to produce primarily xylose and xylobiose. To our knowledge, this is the first report on the characterization of a xylanase from S. cellulosum.