Dietary fiber is an important carbon source for some of the most abundant species of the gut microbiota. In Bacteroidetes, gene clusters encoding protein systems necessary for metabolism of specific complex carbohydrates are colocalized into polysaccharide utilization loci, or PULs. Genomes of Bacteroidetes typically encode several dozen different PULs and transcriptional expression of PULs is strictly regulated. PULs comprise the major nutrient acquisition system for Bacteroidetes and are thus key modulators of microbial ecosystems.

Many PULs are regulated by an unusual class of Two-Component Systems (TCSs) known as Hybrid Two-Component Systems, or HTCSs. In HTCSs, the transmembrane sensor histidine kinase and response regulator transcription factor of canonical TCSs are fused into a single polypeptide chain with extracellular ligand sensing, histidine autophosphorylation, phosphoryl transfer, and transcriptional regulation occurring within a single transmembrane dimer.

The Stock lab at the Center for Advanced Biotechnology and Medicine is applying their expertise in structural and functional analysis of bacterial Two-Component System regulation of gene expression to characterize HTCSs in Bacteroides thetaiotamicron. Initial studies are focused on mechanistic characterization of phosphorylation, dimerization and DNA binding using representative recombinant HTCS proteins. Structural characterization is being pursued by X-ray crystallography and cryo-EM. HTCS signaling mechanisms will be further studied in Bacteroides cells to examine how protein localization impacts signaling.