Two genes encoding thermostable xylanases, named xyn10A and xyn11A, from an alkaliphilic Bacillus firmus were cloned and expressed in Escherichia coli. The E coli harboring either gene showed clear zone with Congo red clearance assay on xylan plate. The Xyn10A and Xyn11A have molecular weights of 45 and 23 kDa, respectively, and both show activities on xylan-zymogram. The xyn10A encodes 396 amino acid residues and is very similar to an alkaliphilic xylanase A from alkaliphilic Bacillus halodurans. The Xyn11A contains 210 amino acid residues and only one amino acid different from an endo-beta-1,4-xylanase from B. halodurans. From alignment of the amino acid sequences with other xylanases, Xyn10A and Xyn11A belong to family 10 and 11 glycosyl hydrolases, respectively. Both show activities over the pH range of 4-11 at 37 degreesC and over 80% activities at 70 degreesC. Interestingly both still retain over 70% activities after 16 h preincubation at 62 degreesC. (C) 2004 Elsevier Inc. All rights reserved.