Borrelia burgdorferi encodes a novel DNA-binding protein in the Fur/PerR family of transcriptional regulators termed BosR (BB0647). This issue of Molecular Microbiology contains two molecular genetic studies that help to clarify the function of BB0647 and resolve longstanding controversies. Loss of BB0647 appears to have a pronounced effect on borrelial gene expression and, in one study, caused significant in vitro growth defects. BB0647 was also found to be essential for infection of the mammalian host but not the tick vector. Both Ouyang et al. and Hyde et al. also demonstrate, quite unexpectedly, that BB0647 is required for induction of RpoS, an alternative sigma factor that controls a cadre of B. burgdorferi genes, most notably ospC, which enable the spirochetes to establish mammalian infection following tick inoculation. There are still many unanswered questions regarding the precise physiological role of BB0647, the most important of which relate to its homologues Fur and PerR: to what extent does it regulate either the response to oxidative stress and/or transition metal uptake? The mechanism(s) whereby BB0647 interfaces with the Rrp2-RpoN-RpoS pathway also remains to be discerned. However, these two seminal papers establish BB0647 (BosR) as a central player in the molecular biology and physiology of B. burgdorferi as well as the pathogenesis of Lyme disease.