Induction of extracellular β-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 b-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 b-galactosidase at comparable growth rates. To know more about the actual inducer for b-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.

Lithium-mediated suppression of morphogenesis and growth in Candida albicans

“…The results (Fig. independent of phosphoglucomutase and different from that described by Leloir such as the one recently reported in Aspergillus nidulans and Hypocrea jecorina (Fekete et al, 2004(Fekete et al, , 2007. As inhibition of phosphoglucomutase by lithium should lead to accumulation of these metabolites, an explanation to these results would be that the phosphorylated hexoses are drained by an alternative pathway such as glycogenesis or the lack of inhibition of phosphoglucomutase in vivo.…”

The effects of disruption of phosphoglucose isomerase gene on carbon utilisation and cellulase production in Trichoderma reesei Rut-C30

“…The enzyme D-xylose reductase, XYL1, converts D-galactose into galactitol [12]. It was formerly shown that galactitol induces β-galactosidase [14,59]. Our results are in agreement with previous data, because the mutant P58, that exhibited a reduced expression of xyl1 , had lower β-galactosidase activity.…”

The d‐xylose reductase of Hypocrea jecorina is the major aldose reductase in pentose and d‐galactose catabolism and necessary for β‐galactosidase and cellulase induction by lactose

“…An alternative hypothesis, i.e. that the xyl1 deletion impairsformation of extracellular β‐galactosidase activity (Fekete et al ., 2007) and thus lactose hydrolysis and hence cellulase induction, can be rejected: cellulase transcript levels have been shown to be formed on lactose independently of the presence of the major β‐galactosidase BGA1 (Seiboth et al ., 2005), and cellulase formation is even improved at low growth rates (Karaffa et al ., 2006). From these data we conclude that the low β‐galactosidase activity itself found in the Δ xyl1 strain is not responsible for the loss of cellulase induction.…”