L -Arabitol Is the Actual Inducer of Xylanase Expression in Hypocrea jecorina (Trichoderma reesei)

Abstract

The saprophytic fungus Hypocrea jecorina (anamorph, Trichoderma reesei) is an important native producer of hydrolytic enzymes, including xylanases. Regarding principles of sustainability, cheap and renewable raw materials, such as D-xylose (the backbone monomer of xylan), have been receiving increasing attention from industries. Recently, it was demonstrated that small (0.5 to 1 mM) amounts of D-xylose induce the highest expression of xylanase in H. jecorina. However, it was also reported that active metabolism of D-xylose is necessary for induction. In this report, we demonstrate that xylitol, the next intermediate in the pentose pathway after D-xylose, does not trigger transcription of xylanase-encoding genes in H. jecorina QM9414. The highest level of transcription of xylanolytic enzyme-encoding genes occurred in an xdh1 (encoding a xylitol dehydrogenase) deletion strain cultured in the presence of 0.5 mM D-xylose, suggesting that a metabolite upstream of xylitol is the inducer. The expression levels of xylanases in an xdh1-lad1 double-deletion strain were lower than that of an xdh1 deletion strain. This observation suggested that L-xylulose is not an inducer and led to the hypothesis that L-arabitol is the actual inducer of xylanase expression. A direct comparison of transcript levels following the incubation of the H. jecorina parental strain with various metabolites of the pentose pathway confirmed this hypothesis. In addition, we demonstrate that xyr1, the activator gene, is not induced in the presence of pentose sugars and polyols, regardless of the concentration used; instead, we observed low constitutive expression of xyr1.

Xylanase Gene Transcription in Trichoderma reesei Is Triggered by Different Inducers Representing Different Hemicellulosic Pentose Polymers

“…These results indicate that the D-xylose reductase step is not essential for xylanase induction, as reported earlier (21). They also show that the actual inducing component that mediates D-xylose induction of the five xylanases is a metabolite accumulating prior to the xylitol oxidation step, as the ⌬lad1/⌬xdh1 double-deletion strain completely blocks further catabolism of xylitol to D-xylulose.…”

“…In T. reesei, the inducing effects by L-arabinose on xylanase gene expression are part of the XYR1 circuit, and future research in aspergilli will show if xylanases are induced by L-arabinose and if this regulation is via XlnR or AraR. Our data also show that this induction is not simply due to the formation of the same inducing metabolite from D-xylose and L-arabinose, as suggested earlier (21), as further supported by the additive effect of induction by D-xylose and L-arabinose. The advantage of having two inducers may come from the fact that there are two major types of xylans: one occurs in the cell wall of cereals, which often contain large quantities of L-arabinose and are consequently termed arabinoxylans.…”

“…The data presented above (i.e., that the absence of D-xylose reductase or xylitol/L-arabitol dehydrogenases leads to increased xylanase gene expression) contradict recent data by Mach-Aigner et al (21,22). One argument to explain these differences is that for transcript analysis they used only a single time point at which expression in the ⌬xyl1 and ⌬xdh1/⌬lad1 strain had not yet taken place and was therefore overlooked.…”

“…4; reviewed in reference 15). Mach-Aigner et al (21,22) studied induction of the xylanase genes xyn1 and xyn2 in T. reesei strains blocked in specific steps in the D-xylose and L-arabinose catabolic pathways. They concluded that the first step in D-xylose catabolism catalyzed by the D-xylose reductase XYL1 is necessary for (full) induction of xyn1 and xyn2 and hypothesized that L-arabitol, formed from D-xylose, would be the true inducer of xylanase expression.…”

Deciphering Transcriptional Regulatory Mechanisms Associated with Hemicellulose Degradation in Neurospora crassa

“…In T. reesei, xylose utilization and xylose reductase are required for full induction of xylanase gene expression (29,30), and a T. reesei strain containing a deletion of xylitol dehydrogenase (xdh1) resulted in an ϳ50% decrease in growth rate on xylose (49). We therefore evaluated whether blocking xylose metabolism in N. crassa would result in feedback to xlr-1 expression.…”