Positive feedback regulation between RpoS and BosR in the Lyme disease pathogen

ABSTRACT In Borrelia burgdorferi , the Lyme disease pathogen, differential gene expression is primarily controlled by the alternative sigma factor RpoS (σ S ). Understanding how RpoS levels are regulated is crucial for elucidating how B. burgdorferi is maintained throughout its enzootic cycle. Our recent studies have shown that a homolog of Fur/PerR repressor/activator, BosR, functions as an RNA-binding protein that controls the rpoS mRNA stability. However, the mechanisms of regulation of BosR, particularly in response to host signals and environmental cues, remain largely unclear. In this study, we revealed a positive feedback loop between RpoS and BosR, where RpoS post-transcriptionally regulates BosR levels. Specifically, mutation or deletion of rpoS significantly reduced BosR levels, while artificial induction of rpoS resulted in a dose-dependent increase in BosR levels. Notably, RpoS does not affect bosR mRNA levels but instead modulates the turnover rate of the BosR protein. Furthermore, we demonstrated that environmental cues do not directly influence bosR expression but instead induce rpoS transcription and RpoS production, thereby enhancing BosR protein levels. This discovery adds a new layer of complexity to the RpoN-RpoS pathway and suggests the need to re-evaluate the factors and signals previously believed to regulate RpoS levels through BosR. IMPORTANCE Lyme disease is the most prevalent arthropod-borne infection in the United States. The etiological agent, Borreliella (or Borrelia ) burgdorferi , is maintained in nature through an enzootic cycle involving a tick vector and a mammalian host. RpoS, the master regulator of differential gene expression, plays a crucial role in tick transmission and mammalian infection of B. burgdorferi . This study reveals a positive feedback loop between RpoS and a Fur/PerR homolog. Elucidating this regulatory network is essential for identifying potential therapeutic targets to disrupt B. burgdorferi ’s enzootic cycle. The findings also have broader implications for understanding the regulation of RpoS and Fur/PerR family in other bacteria.