ABSTRACT The filamentous fungus Trichoderma reesei produces glucuronan lyase (TrGL) when it is grown on β-(1→4)-polyglucuronate (cellouronate) as a sole carbon source. The cDNA encoding TrGL was cloned, and the recombinant enzyme was heterologously expressed in Pichia pastoris . The cDNA of TrGL includes a 777-bp open reading frame encoding a 20-amino-acid signal peptide and the 238-amino-acid mature protein. The amino acid sequence showed no similarity to the amino acid sequences of previously described functional proteins, indicating that the enzyme should be classified in a novel polysaccharide lyase (PL) family. Recombinant TrGL catalyzed depolymerization of cellouronate endolytically by β-elimination and was highly specific for cellouronate. The enzyme was most active at pH 6.5 and 50°C, and its activity and thermostability increased in the presence of Ca 2+ , suggesting that its calcium dependence is similar to that of other PLs, such as pectate lyases.