Lactose metabolism in filamentous fungi: how to deal with an unknown substrate

The disaccharide lactose accumulates as a cheap by-product of the dairy industry and represents therefore a biotechnologically important carbon source for microbial fermentations. Lactose induces cellulolytic enzymes in Hypocrea jecorina (Trichoderma reesei) and is therefore used to produce cellulases and recombinant proteins. This review summarizes our current knowledge on lactose assimilation and regulation with special emphasis on the degradation of the d-galactose moiety in Hypocrea jecorina and Aspergillus nidulans. The GAL(LAC) regulon of the two yeasts Saccharomyces cerevisiae and Kluyveromyces lactis has become a paradigm for transcriptional control in lower eukaryotes and a model system for gene regulation. Our results illustrate that in fungi the gal pathway genes are in general differentially organized and regulated and that fungi degrade d-galactose by at least two different pathways including the classical Leloir pathway and a novel pathway composed of different enzymes of the fungal l-arabinose and d-xylose catabolic pathway. The importance of these pathways for induction of lactose metabolism and cellulases in H. jecorina is discussed.