Lack of aldose 1-epimerase in Hypocrea jecorina (anamorph Trichoderma reesei ): A key to cellulase gene expression on lactose

Abstract

The heterodisaccharide lactose (1,4-O-␤-D-galactopyranosyl-Dglucose) induces cellulase formation in the ascomycete Hypocrea jecorina ‫؍(‬ Trichoderma reesei). Lactose assimilation is slow, and the assimilation of its ␤-D-galactose moiety depends mainly on the operation of a recently described reductive pathway and depends less on the Leloir pathway, which accepts only ␣-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 extracellular medium during growth on lactose. Therefore, the effect of galactomutarotase activity on lactose assimilation was studied with H. jecorina strains expressing the C-terminal galactose mutarotase part of the Saccharomyces cerevisiae Gal10. These strains showed increased growth on lactose in a gene copy numberdependent manner, although their formation of extracellular ␤-galactosidase activity and transcription of the genes encoding the first steps in the Leloir and the reductive pathway was similar to the parental strain QM9414. Cellulase gene transcription on lactose dramatically decreased in these strains, but remained unaffected during growth on cellulose. Our data show that cellulase induction in H. jecorina by lactose requires the ␤-anomer of D-galactose and reveal the lack of mutarotase activity during growth on lactose as an important key for cellulase formation on this sugar. mutarotase ͉ Leloir pathway ͉ reductive D-galactose catabolism

Identification of Major Facilitator Transporters Involved in Cellulase Production during Lactose Culture ofTrichoderma reeseiPC-3-7

“…Much of the research on cellulase induction by lactose has focused on the metabolism of lactose. 34,56,57) Despite observations that indicate that lactose is a stronger inducer than Dgalactose, 33) the importance of lactose permeases on cellulase formation has not attracted widespread attention. In this study, we identified two major facilitator superfamily sugar transporters involved in lactose uptake and cellulase induction by lactose in an industrial strain of T. reesei.…”

Reduced genomic potential for secreted plant cell-wall-degrading enzymes in the ectomycorrhizal fungus Amanita bisporigera, based on the secretome of Trichoderma reesei

“…A plausible explanation for this is the use of lactose, the substrate of b-galactosidase, for induction of cellulase gene expression in commercial fermentations (Fekete et al, 2008). Corn stover contains little if any b-linked galactose.…”

Properties of a major β-glucosidase-BGL1 from Aspergillus niger NII-08121 expressed differentially in response to carbon sources

“…It supports the glucose tolerant BGL production and it is utilized in the commercial production of the enzyme owing to economic considerations [10]. The mechanism of lactose induction, however, is not fully understood [11].…”