Abstract
BosR, the sole member of the ferric uptake regulator (FUR) family in Borrelia burgdorferi, is essential for the spirochete's transcriptional adaptation to the mammalian host environment. Although best known for activating rpoS and establishing the mammalian-phase RpoS regulon, BosR originally was linked to regulation of genes involved in B. burgdorferi's oxidative stress response. Here, we show that BosR governs gene expression through both RpoS-dependent and RpoS-independent mechanisms under in vitro and mammalian host-adapted conditions. Using RNA-seq and a DNA-binding-defective BosR-R39A mutant, we demonstrate that DNA binding is essential for BosR's global regulatory functions. BosR activates rpoS, promotes RpoS-dependent gene regulation, and independently modulates a distinct set of genes involved in a variety of cellular functions, including genome maintenance, chemotaxis, and virulence. Notably, canonical oxidative stress response genes previously attributed to BosR were not differentially expressed in ΔbosR strains in vitro or in mammals. Despite its broad regulatory scope, BosR does not recognize a single, conserved DNA-binding motif, suggesting that DNA occupancy is influenced by local sequence context or DNA topology. Our findings support a bifunctional model in which BosR collaborates with RNA polymerase (RNAP)-RpoS holoenzyme to activate and repress RpoS-regulated genes, while functioning in a FUR-like manner to control RpoD-dependent genes independently of RNAP interaction.