Handling Several Sugars at a Time: a Case Study of Xyloglucan Utilization by Ruminiclostridium cellulolyticum

Xyloglucan utilization by Ruminiclostridium cellulolyticum was formerly shown to imply the uptake of large xylogluco-oligosaccharides, followed by cytosolic depolymerization into glucose, galactose, xylose, and cellobiose. This raises the question of how the anaerobic bacterium manages the simultaneous presence of multiple sugars. Using genetic and biochemical approaches targeting the corresponding metabolic pathways, we observed that, surprisingly, all sugars are catabolized, collectively, but glucose consumption is prioritized. Most selected enzymes display unusual features, especially the GTP-dependent hexokinase of glycolysis, which appeared reversible and crucial for xyloglucan utilization. In contrast, mutant strains lacking either galactokinase, cellobiose-phosphorylase, or xylulokinase still catabolize xyloglucan but display variably altered growth. Furthermore, the xylogluco-oligosaccharide depolymerization process appeared connected to the downstream pathways through an intricate network of competitive and noncompetitive inhibitions. Altogether, our data indicate that xyloglucan utilization by R. cellulolyticum relies on an energy-saving central carbon metabolism deviating from current bacterial models, which efficiently prevents carbon overflow. IMPORTANCE The study of the decomposition of recalcitrant plant biomass is of great interest as the limiting step of terrestrial carbon cycle and to produce plant-derived valuable chemicals and energy. While extracellular cellulose degradation and catabolism have been studied in detail, few publications describe the complete metabolism of hemicelluloses and, to date, the published models are limited to the extracellular degradation and sequential entry of simple sugars. Here, we describe how the model anaerobic bacterium Ruminiclostridium cellulolyticum deals with the synchronous intracellular release of glucose, galactose, xylose, and cellobiose upon cytosolic depolymerization of imported xyloglucan oligosaccharides. The described novel metabolic strategy involves the simultaneous activity of different metabolic pathways coupled to a network of inhibitions controlling the carbon flux and is distinct from the ubiquitously observed sequential uptake and metabolism of carbohydrates known as the diauxic shift. Our results highlight the diversity of cellular responses related to a complex environment.