The L-Arabinan utilization system of Geobacillus stearothermophilus

Geobacillus stearothermophilus T-6 is a thermophilic soil bacterium that has a 38-kb gene cluster for the utilization of arabinan, a branched polysaccharide that is part of the plant cell wall. The bacterium encodes a unique three-component regulatory system (araPST) that includes a sugar-binding lipoprotein (AraP), a histidine sensor kinase (AraS), and a response regulator (AraT) and lies adjacent to an ATP-binding cassette (ABC) arabinose transport system (araEGH). The lipoprotein (AraP) specifically bound arabinose, and gel mobility shift experiments showed that the response regulator, AraT, binds to a 139-bp fragment corresponding to the araE promoter region. Taken together, the results showed that the araPST system appeared to sense extracellular arabinose and to activate a specific ABC transporter for arabinose (AraEGH). The promoter regions of the arabinan utilization genes contain a 14-bp inverted repeat motif resembling an operator site for the arabinose repressor, AraR. AraR was found to bind specifically to these sequences, and binding was efficiently prevented in the presence of arabinose, suggesting that arabinose is the molecular inducer of the arabinan utilization system. The expression of the arabinan utilization genes was reduced in the presence of glucose, indicating that regulation is also mediated via a catabolic repression mechanism. The cluster also encodes a second putative ABC sugar transporter (AbnEFJ) whose sugar-binding lipoprotein (AbnE) was shown to interact specifically with linear and branched arabino-oligosaccharides. The final degradation of the arabino-oligosaccharides is likely carried out by intracellular enzymes, including two α-l-arabinofuranosidases (AbfA and AbfB), a β-l-arabinopyranosidase (Abp), and an arabinanase (AbnB), all of which are encoded in the 38-kb cluster.