The nucleotide sequence of the xynA gene, encoding extracellular xylanase A of Thermotoga neapolitana, was determined. The xynA gene was 3264 base pairs (bp) long and encoded a putative polypeptide of 1055 amino acids. Three different domains were identified by sequence comparison and functional analysis of proteins with N- and/or C-terminal deletions. The core domain displayed significant homology to members of the glycosyl hydrolase family 10. N- and C-terminal domains were dispensable for enzymatic activity and seemed to be responsible for thermostability and cellulose binding, respectively. The intact gene and its truncated variants were expressed in Escherichia coli and purified for biochemical characterization. The enzyme was shown to act as an endo-1,4-beta-xylanase, but minor activities against lichenan, barley glucan, methylumbelliferyl cellobioside and p-nitrophenyl xyloside were also detected. The specific activity and pH and temperature optima for hydrolysis of oat xylan were 111.3 U . mg(-1), 5.5 and 102 degrees C, respectively. The endoxylanase was stable at 90 degrees C and retained 50% activity when incubated for 2 h at 100 degrees C.