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The heterodisaccharide lactose (1,4-O-β-d-galactopyranosyl-d-glucose) induces cellulase formation in the ascomycete Hypocrea jecorina (= Trichoderma reesei). Lactose assimilation is slow, and the assimilation of its β-d-galactose moiety depends mainly on the operation of a recently described reductive pathway and depends less on the Leloir pathway, which accepts only α-d-galactose. We therefore reasoned whether galactomutarotase [aldose 1-epimerase (AEP)] activity might limit lactose assimilation and thus influence cellulase formation. We identified three putative AEP-encoding genes (aep1, aep2, aep3) in H. jecorina, of which two encoded intracellular protein (AEP1 and AEP2) and one encoded an extracellular protein (AEP3). Although all three were transcribed, only the aep3 transcript was detected on lactose. However, no mutarotase activity was detected in the mycelia, their cell walls, or the …