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The galactokinase of Hypocrea jecorina is essential for cellulase induction by lactose but dispensable for growth on d ‐galactose
Author(s) -
Seiboth Bernhard,
Hartl Lukas,
Pail Manuela,
Fekete Erzsébet,
Karaffa Levente,
Kubicek Christian P.
Publication year - 2004
Publication title -
molecular microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.857
H-Index - 247
eISSN - 1365-2958
pISSN - 0950-382X
DOI - 10.1046/j.1365-2958.2003.03901.x
Subject(s) - hypocrea , galactokinase , galactitol , biochemistry , lactose , galactose , biology , lac repressor , trichoderma reesei , kluyveromyces lactis , cellulase , mutant , saccharomyces cerevisiae , yeast , repressor , escherichia coli , gene , enzyme , gene expression
Summary Lactose is the only soluble carbon source which can be used economically for the production of cellulases or heterologous proteins under cellulase expression signals by Hypocrea jecorina (= Trichoderma reesei ). Towards an understanding of lactose metabolism and its role in cellulase formation, we have cloned and characterized the gal1 (galactokinase) gene of H. jecorina , which catalyses the first step in d ‐galactose catabolism. It exhibits a calculated M r of 57 kDa, and shows moderate identity (about 40%) to its putative homologues of Saccharomyces cerevisiae and Kluyveromyces lactis . Gal1 is a member of the GHMP family, shows conservation of a Gly/Ser rich region involved in ATP binding and of amino acids (Arg 51, Glu 57, Asp 60, Asp 214, Tyr 270) responsible for galactose binding. A single transcript was formed constitutively during the rapid growth phase on all carbon sources investigated and accumulated to about twice this level during growth on d ‐galactose, l ‐arabinose and their corresponding polyols. Deletion of gal1 reduces growth on d ‐galactose but does only slightly affect growth on lactose. This is the result of the operation of a second pathway for d ‐galactose catabolism, which involves galactitol as an intermediate, and whose transient concentration is strongly enhanced in the delta‐ gal1 strain. In this pathway, galactitol is catabolised by the lad1 ‐encoded l ‐arabinitol‐4‐dehydrogenase, because a gal1 / lad1 double delta‐mutant failed to grow on d ‐galactose. In the delta‐ gal1 strain, induction of the Leloir pathway gene gal7 (encoding galactose‐1‐phosphate uridylyltransferase) by d ‐galactose, but not by l ‐arabinose, is impaired. Induction of cellulase gene expression by lactose is also impaired in a gal1 deleted strain, whereas their induction by sophorose (the putative cellulose‐derived inducer) was shown to be normal, thus demonstrating that galactokinase is a key enzyme for cellulase induction during growth on lactose, and that induction by lactose and sophorose involves different mechanisms.