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1 Department of Microbiology and Biotechnology, Faculty of Sciences, University of Debrecen, H-4010, PO Box 63, Debrecen, Hungary
2 Research Area Gene Technology and Applied Biochemistry, Institute of Chemical Engineering, TU Wien, Getreidemarkt 9/1665, A-1060 Wien, Austria
Correspondence
Bernhard Seiboth
bseiboth{at}mail.zserv.tuwien.ac.at
Lactose (1,4-O-
-D-galactopyranosyl-D-glucose) is a soluble and economic carbon source for the industrial production of cellulases or recombinant proteins by Hypocrea jecorina (anamorph Trichoderma reesei). The mechanism by which lactose induces cellulase formation is not understood. Recent data showed that the galactokinase step is essential for cellulase induction by lactose, but growth on D-galactose alone does not induce cellulases. Consequently, the hypothesis was tested that D-galactose may be an inducer only at a low growth rate, which is typically observed when growing on lactose. Carbon-limited chemostat cultivations of H. jecorina were therefore performed at different dilution rates with D-galactose, lactose, galactitol and D-glucose. Cellulase gene expression was monitored by using a strain carrying a fusion between the cbh2 (encoding cellobiohydrolase 2, Cel6A) promoter region and the Aspergillus niger glucose oxidase gene and by identification of the two major cellobiohydrolases Cel7A and Cel6A. The results show that D-galactose indeed induces cbh2 gene transcription and leads to Cel7A and Cel6A accumulation at a low (D=0·015 h–1) but not at higher dilution rates. At the same dilution rate, growth on D-glucose did not lead to cbh2 promoter activation or Cel6A formation but a basal level, lower than that observed on D-galactose, was detected for the carbon-catabolite-derepressible Cel7A. Lactose induced significantly higher cellulase levels at 0·015 h–1 than D-galactose and induced cellulases even at growth rates up to 0·042 h–1. Results of chemostats with an equimolar mixture of D-galactose and D-glucose essentially mimicked the behaviour on D-galactose alone, whereas an equimolar mixture of D-galactose and galactitol, the first intermediate of a recently described second pathway of D-galactose catabolism, led to cellulase induction at D=0·030 h–1. It is concluded that D-galactose indeed induces cellulases at low growth rate and that the operation of the alternative pathway further increases this induction. However, under those conditions lactose is still a superior inducer for which the mechanism remains to be clarified.
This article has been cited by other articles:
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  E. Fekete, B. Seiboth, C. P. Kubicek, A. Szentirmai, and L. Karaffa Lack of aldose 1-epimerase in Hypocrea jecorina (anamorph Trichoderma reesei): A key to cellulase gene expression on lactose PNAS, May 20, 2008; 105(20): 7141 - 7146. [Abstract] [Full Text] [PDF]
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 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]
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 E. Fekete, L. Karaffa, C. P. Kubicek, A. Szentirmai, and B. Seiboth Induction of extracellular beta-galactosidase (Bga1) formation by D-galactose in Hypocrea jecorina is mediated by galactitol Microbiology, February 1, 2007; 153(2): 507 - 512. [Abstract] [Full Text] [PDF]
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