An arabinoxylan arabinofuranohydrolase (AXS5) was purified from the culture filtrate of Penicillium chrysogenum 31B. A cDNA encoding AXS5 (axs5) was isolated by in vitro cloning using the N-terminal amino acid sequence of the native enzyme as a starting point. The deduced amino acid sequence of the axs5 gene has high similarities with those of arabinoxylan arabinofuranohydrolases of Aspergillus niger, Aspergillus tubingensis, and Aspergillus sojae. Module sequence analysis revealed that a "Glyco_hydro_62" was present at position 28-299 of AXS5. This is a family of α-L-arabinofuranosidases which are all members of glycoside hydrolase family 62. Recombinant AXS5 (rAXS5) expressed in Escherichia coli was highly active on arabinoxylan but not on branched sugar beet arabinan. (1)H-NMR analysis revealed that the rAXS5 cleaved arabinosyl side-chains linked to C-2 and C-3 of single-substituted xylose residues in arabinoxylan. Semi-quantitative RT-PCR analysis indicated that expression of the axs5 gene in P. chrysogenum 31B was strongly induced by adding D-xylose and arabinoxylan to the culture medium. Moreover, two binding sites of XlnR, a transcriptional activator that regulates the expression of the genes encoding xylanolytic enzymes, are present in the upstream region of the axs5 gene. These results suggest that AXS5 is involved in xylan degradation.