The cellulosome of Clostridium thermocellum is a multiprotein complex with endo-and exocellulase, xylanase, -glucanase, and acetyl xylan esterase activities. XynY and XynZ, components of the cellulosome, are composed of several domains including xylanase domains and domains of unknown function (UDs). Database searches revealed that the C-and N-terminal UDs of XynY and XynZ, respectively, have sequence homology with the sequence of a feruloyl esterase of strain PC-2 of the anaerobic fungus Orpinomyces. Purified cellulosomes from C. thermocellum were found to hydrolyze
FAXX (O-{5-O-[(E)-feruloyl]-␣-L-arabinofuranosyl}-(133)-O--D-xylopyranosyl-(134)-D-xylopyranose) and FAX 3 (5-O-[(E)-feruloyl]-[O--D-xylopyranosyl-(132)]-O-␣-L-arabinofuranosyl-[133]}-O--D-xylopyranosyl-(