The identification and characterization of unique FemX homologue in B. burgdorferi, and insights into the peptidoglycan biosynthesis pathway

Borrelia burgdorferi — the causative agent of Lyme borreliosis — accounts for ~500,000 infections in the United States per year. Relative to other bacteria, B. burgdorferi is highly unusual in many regards. For instance, the synthesis and composition of B. burgdorferi cell wall is extremely unique and plays a critical role in Lyme pathogenesis. The cell wall is made up of peptidoglycan (PG) - a mesh-like structure, composed of long rigid glycan strands of repeating sugars GlcNAc and MurNAc, and flexible peptide stems, interlinked by amino acid cross-bridges. PG is an essential component for survival of the bacterial cell, protecting it from the osmotic stress and environmental threats, as well as defining the shape of the bacterium and aiding in the motility. One unique feature of the B. burgdorferi PG is the chemical composition of stem peptide, which involves the atypical cross-link between Ornithine and Glycine. We identified gene bb0586 as a femX homologue in borrelial genome and hypothesize that it encodes a glycyl transferase enzyme responsible for synthesis of glycine cross-bridges, that hold together glycan strands in the peptidoglycan cell wall. Here, we predicted the structure of FemXBb, identified and characterized the substrate-binding site, and proposed a novel mechanism for substrate recognition and recruitment, involving previously uncharacterized elements of the structure. We have also determined the ability of recombinant FemXBb to add Glycine bridges to mDAP in E. coli and investigated the effect that femX knock-out can have on the B. burgdorferi. In addition, we have investigated the steps of PG biosynthesis in B. burgdorferi. The results of our research suggest the existence of a highly unusual mechanism of PG synthesis in Lyme disease spirochete, which has a potential to be used for development of targeted antibacterial therapies.