Abstract
CDT-1 and CDT-2 are two cellodextrin transporters discovered in the filamentous fungus Neurospora crassa. Previous studies focused on characterizing the role of these transporters in only a few conditions, including cellulose degradation, and the function of these two transporters is not yet completely understood. In this study, we show that deletion of cdt-2, but not cdt-1, results in growth defects not only on Avicel but also on xylan. cdt-2 can be highly induced by xylan, and this mutant has a xylodextrin consumption defect. Transcriptomic analysis of the cdt-2 deletion strain on Avicel and xylan showed that major cellulase and hemicellulase genes were significantly down-regulated in the cdt-2 deletion strain and artificial over expression of cdt-2 in N. crassa increased cellulase and hemicellulase production. Together, these data clearly show that CDT-2 plays a critical role in hemicellulose sensing and utilization. This is the first time a sugar transporter has been assigned a function in the hemicellulose degradation pathway. Furthermore, we found that the transcription factor XLR-1 is the major regulator of cdt-2, while cdt-1 is primarily regulated by CLR-1. These results deepen our understanding of the functions of both cellodextrin transporters, particularly for CDT-2. Our study also provides novel insight into the mechanisms for hemicellulose sensing and utilization in N. crassa, and may be applicable to other cellulolytic filamentous fungi.
Highlights
Lignocellulose degradation is a critical step for biofuels and biobased chemical production in biorefineries [1,2]
CDT-1 and Cellodextrin Transporte 2 (CDT-2) were identified as cellodextrin transporters [11]
No growth of the double knock out strain on Avicel was observed and its conidia did not germinate. These results are consistent with previous data that cdt-1 and cdt-2 are both cellodextrin transporters, with redundant functions in cellobiose and cellodextrin transport [11]
Summary
Lignocellulose degradation is a critical step for biofuels and biobased chemical production in biorefineries [1,2]. Lignocellulases are expressed at a basal level on the periphery of conidia, and these enzymes degrade biomass into various carbohydrates [3,4] These carbohydrates include: cellodextrins (glucose polymers, such as cellobiose, cellotriose and cellotetraose) derived from cellulose, and xylodextrins (xylose polymers, such as xylobiose, xylotriose and xylotetraose) derived from hemicellulose [5,6,7]. Oligosaccharides such as cellodextrins and xylodextrins are thought to function as inducer molecules. Two major facilitator superfamily (MFS) sugar transporters, Stp and Crt, were implicated in cellulose sensing and cellulase induction in T. reesei [12]. Sugar transporters involved in sensing hemicellulose have not been previously reported
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