Carbohydrate catabolite repression is functionally redundant for the fermentation of lignocellulose-derived sugars by Thermoanaerobacterium thermosaccharolyticum

Abstract

The phosphotransferase system (PTS) is a conserved mechanism in most bacteria, responsible for carbohydrate transport and regulation. This system plays a key role in carbon catabolite repression (CCR), a regulatory process that prioritizes the metabolism of preferred sugars. However, PTS and CCR have received little attention in fermentations of oligomeric lignocellulose-derived carbohydrates. Thermoanaerobacterium thermosaccharolyticum, a thermophilic gram-positive anaerobic bacterium capable of fermenting a diversity of simple and complex carbohydrates, is a promising candidate for lignocellulosic ethanol production. This study investigated the role of PTS and CCR by deleting the ptsH gene in T. thermosaccharolyticum. Monomeric sugar utilization experiments revealed that deletion of ptsH resulted in the loss of selective sugar utilization by effectively disrupting CCR. Assays with 2-deoxyglucose and cyclic adenosine monophosphate (cAMP) confirmed that ptsH is involved in the T. thermosaccharolyticum CCR response and is cAMP dependent. Its deletion alleviates catabolite repression, especially for cellobiose. Remarkably, fermentation of hemicellulose-derived carbohydrates from sugarcane bagasse spent broth was not affected by deletion of the ptsH gene, indicating that CCR has minimal impact under lignocellulosic carbohydrate substrate conditions. This research provides new insights into sugar catabolism regulation in T. thermosaccharolyticum, and for the first time demonstrates that CCR is not important when fermenting lignocellulose-derived sugars.