A SPECIFIC OLIGOPEPTIDE TRANSPORTER MEDIATES
QUORUM-SENSING REGULATION OF THE EXTRACELLULAR XYLANASE GENE IN GEOBACILLUS STEAROTHERMOPHILUS

Geobacillus stearothermophilus T-6 possesses a single extracellular xylanase (Xyn10A), capable of producing short, decorated xylo-oligosaccharides from the naturally branched polysaccharide, xylan. The specific activity of the extracellular xylanase increases over 10-fold during early exponential growth, suggesting cell density regulation (quorum sensing). Addition of conditioned medium to low cell density cultures resulted in high expression of xynA, indicating that a diffusible extracellular xynA density factor (XDF) is present in the medium. XDF is heat-stable, sensitive to proteases and was partially purified using reverse phase liquid chromatography. Based on these results, it is likely that XDF is a small hydrophobic peptide or peptides. Secreted extracellular signaling peptides can be imported to the cell via specific oligopeptide (Opp) transport systems.
Based on its genome sequence, G. stearothermophilus T-6 possesses a single Opp transport system composed of five genes (oppABCDF). Bialaphos is a toxic tri-peptide that is known to enter bacteria via oligopeptide permeases. We have isolated a bialaphos-resistant mutant of G. stearothermophilus, and found that the oppB gene is interrupted by an insertion element and presumably lacks a functional Opp transport system. In this bialaphos-resistant mutant the xynA gene does not appear to be regulated by cell density, suggesting that an intact oligopeptide transport system is required for quorum-sensing regulation of the xynA gene.