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Induction of extracellular -galactosidase (Bga1) formation by D-galactose in Hypocrea jecorina is mediated by galactitol

¹ Department of Genetics and Applied Microbiology, Faculty of Science, University of Debrecen, H-4010, PO Box 56, Debrecen, Hungary
² Research Area Gene Technology and Applied Biochemistry, Institute of Chemical Engineering, TU Wien, Getreidemarkt 9/166-5, A-1060 Wien, Austria

Correspondence
Bernhard Seiboth
bseiboth{at}mail.zserv.tuwien.ac.at

The ability of Hypocrea jecorina (Trichoderma reesei) to grow on lactose strongly depends on the formation of an extracellular glycoside hydrolase (GH) family 35 -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 -galactosidase at comparable growth rates. To know more about the actual inducer for -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.

This article has been cited by other articles:

				L. Hartl, C. P. Kubicek, and B. Seiboth Induction of the gal Pathway and Cellulase Genes Involves No Transcriptional Inducer Function of the Galactokinase in Hypocrea jecorina J. Biol. Chem., June 22, 2007; 282(25): 18654 - 18659. [Abstract] [Full Text] [PDF]