The degradation of cellulose and hemicellulose (the main components of the plant cell wall) is a pivotal step in the carbon cycle on Earth. This process is mediated mainly by microorganisms which are found either free in nature or as part of the digestive tracts of higher animals. Geobacillus stearothermophilus is a thermophilic aerobic soil bacterium that possesses an extensive enzymatic system for the degradation of hemicellulose. Many of the catalytic components of its hemicellulolytic system have been studied extensively and their high resolution crystal structures, substrate specificities and catalytic mechanisms have been determined. The regulation of the hemicellulolytic system in G. stearothermophilus is intricate and involves novel features such as quorum sensing at low cell densities and substrate-sensing regulatory systems. The biological logic of regulating vegetative enzymes at low cell densities may be related to cell-density dependent growth phenomenon on the high molecular weight substrate, xylan. Results by our group indicate that the hemicellulolytic system in G. stearothermophilus possesses noteworthy features that allow the bacterium to efficiently compete for the scarce carbon in the soil. These features include dedicated oligosaccharides sensing and transport systems, quorum sensing regulation at low cell densities and highly proficient hemicellulolytic enzymes.
