Biomass is considered a readily viable solution as a renewable energy source for liquid fuel (bio-ethanol) that does not contribute net CO2to the atmosphere. The two main challenges in converting biomass to liquid fuel are the economic conversion cellulose to glucose, and the utilization of five-carbon sugars by fermenting microorganisms. To engineer fermenting microorganisms capable of utilizing pentoses it is possible to use pentose-related functions taken fromhemicellulolyticmicroorganisms, such as the metabolic enzymes and various sugar transporters.Geobacillus stearothermophilusutilizes an extensive and highly regulated hemicellulolytic system that is based on three extracellular glycoside hydrolases, xylanase, arabinanase and galactanase, which partially hydrolyze the main backbone to give decorated oligosaccharides. The uptake of these oligosaccharides is mediated by five dedicatedATP-bindingcassette(ABC) sugar transporters and the final breakdown is carried out by intracellular enzymes.
Based on sequence homology, we purified the sugar-binding proteins and by using microcalorimetric studies we showed that binding-proteins can specifically interact with various oligosaccharides. Regulation studies indicate that the ABC transporter for xylo-oligosaccharides is activated by a response regulator, which is part of a two-component sensing system. The transporter system for arabinose is regulated by a unique three-component sensing system which composed of an arabinose receptor, histidine kinase and a response regulator. These sensing systems allow the cell to respond to very low concentrations of carbohydrates in the soil and efficiently uptake these oligosaccharides. Genome sequence analysis revealed that the ATPase component of the ABC transporters is probably common to all transporter systems.
