| Two-component systems (TCS) are universal among bacteria and play critical roles in global regulation. The contribution of TCS to the biology and pathogenesis of Borrelia are poorly understood. Borrelia burgdorferi, a causative agent of Lyme disease, harbors only two TCS, one of which is comprised of Rrp1, a response regulator and Hpk1, a histidine kinase. In the Lyme Borrelia, Rrp1 proteins, which contain GG(D/E)EF domains, undergo phosphorylation and produce the secondary messenger, cyclic diguanylate (c-di-GMP). Cyclic-di-GMP is a critical signaling molecule in many bacteria. However, in Borrelia the regulatory roles of TCS and c-di-GMP are largely unexplored. In this study, the distribution, conservation and expression of Rrp1 was assessed, and its potential contribution to the pathobiology of the Lyme Borrelia was examined through allelic exchange mutagenesis and subsequent microarray analyses. Interestingly, the majority of genes regulated by Rrp1 are carried on genetic elements comprising the core Borrelia genome (chromosome, lp54 and cp26). Rrp1 regulates a large number of genes that encode for proteins with diverse functions, including several virulence factors such as factor H binding proteins (FHBPs). Like Borrelia, many pathogens have evolved to bind FH to their surfaces to aid in immune evasion by inhibiting the progression of the alternative pathway of complement. In this study, we also examined the conservation, distribution, properties, and ligand binding abilities of the B. burgdorferi FHBP, CspZ. CspZ is highly conserved, and all CspZ orthologs expressed by B. burgdorferi isolates bind FH while CspZ homologs expressed by B. garinii and B. afzelii isolates do not. Regardless of the Borrelia species, all CspZ proteins tested bind to unknown ∼60 kDa serum proteins from various animals. Typical of many FHBPs, structural analyses revealed that the key determinants required for ligand binding are discontinuous and dependent on coiled-coil structures. The data presented within are the first to describe Rrp1-mediated gene regulation and the tremendous impact this TCS has on Borrelia biology and pathogenesis. Additionally, the analyses of CspZ have added to our understanding of complement evasion and possible alternative functions of the Borrelia FHBPs and may serve as a diagnostic tool for Lyme disease. |
