Hydrogenomics of the extremely thermophilic bacterium Caldicellulosiruptor saccharolyticus

Caldicellulosiruptor saccharolyticus is an extremely thermophilic, gram-positive anaerobe which ferments
cellulose-, hemicellulose- and pectin-containing biomass to acetate, CO2, and hydrogen. Its broad substrate
range, high hydrogen-producing capacity, and ability to coutilize glucose and xylose make this bacterium an
attractive candidate for microbial bioenergy production. Here, the complete genome sequence of C. saccharolyticus,
consisting of a 2,970,275-bp circular chromosome encoding 2,679 predicted proteins, is described.
Analysis of the genome revealed that C. saccharolyticus has an extensive polysaccharide-hydrolyzing capacity
for cellulose, hemicellulose, pectin, and starch, coupled to a large number of ABC transporters for monomeric
and oligomeric sugar uptake. The components of the Embden-Meyerhof and nonoxidative pentose phosphate
pathways are all present; however, there is no evidence that an Entner-Doudoroff pathway is present. Catabolic
pathways for a range of sugars, including rhamnose, fucose, arabinose, glucuronate, fructose, and galactose,
were identified. These pathways lead to the production of NADH and reduced ferredoxin. NADH and reduced
ferredoxin are subsequently used by two distinct hydrogenases to generate hydrogen. Whole-genome transcriptome
analysis revealed that there is significant upregulation of the glycolytic pathway and an ABC-type sugar
transporter during growth on glucose and xylose, indicating that C. saccharolyticus coferments these sugars
unimpeded by glucose-based catabolite repression. The capacity to simultaneously process and utilize a range
of carbohydrates associated with biomass feedstocks is a highly desirable feature of this lignocelluloseutilizing,
biofuel-producing bacterium.