title: Induction of extracellular beta-galactosidase (Bga1) formation by D-galactose in Hypocrea jecorina is mediated by galactitol.
abstract: The ability of Hypocrea jecorina (Trichoderma reesei) to grow on lactose strongly depends on the formation of an extracellular glycoside hydrolase (GH) family 35 beta-galactosidase, encoded by the bga1 gene. Previous studies, using batch or transfer cultures of pregrown cells, had shown that bga1 is induced by lactose and d-galactose, but to a lesser extent by galactitol. To test whether the induction level is influenced by the different growth rates attainable on these carbon sources, bga1 expression was compared in carbon-limited chemostat cultivations at defined dilution (=specific growth) rates. The data showed that bga1 expression by lactose, d-galactose and galactitol positively correlated with the dilution rate, and that galactitol and d-galactose induced the highest activities of beta-galactosidase at comparable growth rates. To know more about the actual inducer for beta-galactosidase formation, its expression in H. jecorina strains impaired in the first steps of the two d-galactose-degrading pathways was compared. Induction by d-galactose and galactitol was still found in strains deleted in the galactokinase-encoding gene gal1, which is responsible for the first step of the Leloir pathway of d-galactose catabolism. However, in a strain deleted in the aldose/d-xylose reductase gene xyl1, which performs the reduction of d-galactose to galactitol in a recently identified second pathway, induction by d-galactose, but not by galactitol, was impaired. On the other hand, induction by d-galactose and galactitol was not affected in an l-arabinitol 4-dehydrogenase (lad1)-deleted strain which is impaired in the subsequent step of galactitol degradation. These results indicate that galactitol is the actual inducer of Bga1 formation during growth on d-galactose in H. jecorina.
authors.affiliation: Department of Genetics and Applied Microbiology, Faculty of Science, University of Debrecen, H-4010, PO Box 56, Debrecen, Hungary.
authors.forename: Erzsébet
authors.initials: E
authors.lastname: Fekete
authors.affiliation: Department of Genetics and Applied Microbiology, Faculty of Science, University of Debrecen, H-4010, PO Box 56, Debrecen, Hungary.
authors.forename: Levente
authors.initials: L
authors.lastname: Karaffa
authors.affiliation: Research Area Gene Technology and Applied Biochemistry, Institute of Chemical Engineering, TU Wien, Getreidemarkt 9/166-5, A-1060 Wien, Austria.
authors.forename: Christian P
authors.initials: CP
authors.lastname: Kubicek
authors.affiliation: Department of Genetics and Applied Microbiology, Faculty of Science, University of Debrecen, H-4010, PO Box 56, Debrecen, Hungary.
authors.forename: Attila
authors.initials: A
authors.lastname: Szentirmai
authors.affiliation: Research Area Gene Technology and Applied Biochemistry, Institute of Chemical Engineering, TU Wien, Getreidemarkt 9/166-5, A-1060 Wien, Austria.
authors.forename: Bernhard
authors.initials: B
authors.lastname: Seiboth
mesh_terms: D003470:Culture Media; D005656:Fungal Proteins / Q000235:genetics / Q000378:metabolism; D004376:Galactitol / Q000378:metabolism*; D005690:Galactose / Q000378:metabolism*; D015971:Gene Expression Regulation, Enzymologic; D015966:Gene Expression Regulation, Fungal; D005947:Glucose / Q000378:metabolism; D035901:Hypocrea / Q000201:enzymology* / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D001616:beta-Galactosidase / Q000235:genetics / Q000378:metabolism*
chemical_list: D003470:Culture Media; D005656:Fungal Proteins; D004376:Galactitol; D001616:beta-Galactosidase; D005947:Glucose; D005690:Galactose