Talaromyces emersonii produces a complete xylan-degrading enzyme system when grown on appropriate substrates (Tuohy & Coughlan, 1992). The individual components are all markedly thermostable glycoproteins with half-life values at pH 4-5 in excess of 8 days at 50-degrees-C and, depending on the enzyme, from 5-250 min at 80-degrees-C. The system includes at least 13 endo-beta-1,4-D-xylanases (EC 3.2.1.8), one beta-D-xylosidase (EC 3.2.1.37), one alpha-L-arabinofuranosidase (EC 3.2.1.55), three acetylxylan esterases, while alpha-D-glucuronidase, ferulic and p-coumaric acid esterase activities are also present. Most of these enzymes have been purified to apparent homogeneity and characterized as individual components and in various combinations. The xylanases have M(r) values ranging from 57000-131000, pI values from pH 3.5-5.5, pH optima from pH 3.5-4.7, temperature optima from 67-80-degrees-C. K(m) values with the soluble fraction of oat spelts xylan as substrate range from 0.4 to 13.3 mg ml-1 and k(cat) values from 216-9213 s-1. All but two of the xylanases are active against a variety of xylooligosaccharides and against substituted and unsubstituted xylans (Tuohy et al., 1993a; b). Products of hydrolysis of a variety of xylooligosaccharides, substituted and unsubstituted xylans vary with the substrate and the enzyme used. There is evidence of transferase, as well direct hydrolase, activity in the case of several components. The two unusual xylanases catalyze the hydrolysis of artificial xylosides, xylooligosaccharides and unsubstituted xylans. Prior removal of arabinosyl residues is a prerequisite to activity against wheat-straw arabinoxylan. The beta-xylosidase is a dimer (M(r) of 181000; pI 8.9) with pH and temperature optima of 2.5 and 60-degrees-C, respectively. The K(m) value with pNPX as substrate is 0.13 mM and the k(cat) is 426 s-1. It is active against p- and chloronitrophenyl-beta-xylosides and against unsubstituted xylooligosaccharides from X1 to X7 with X1 being the main end-product of hydrolysis. Activity decreases with increasing DP and there is little or no activity against polymeric xylan. The alpha-arabinofuranosidase is a dimer (M(r) 210000) with pH and temperature optima of 3.2 and 70-degrees-C, respectively. The K(m) value with pNPA as substrate, is 0.16 mM and the k(cat) is approximately 10.5 s-1. Arabinose is the sole end-product of long-term hydrolysis of arabinoxylan. Examples of homeo- and heterosynergistic reactions (Coughlan et al., 1993) have been noted between the various components in the hydrolysis of xylans.