bb0689 contributes to the virulence of Borrelia burgdorferi in a murine model of Lyme disease.

Borrelia burgdorferi, the Lyme disease pathogen, continuously changes its gene expression profile in order to adapt to ticks and mammalian hosts. The alternative sigma factor RpoS plays a central role in borrelial host adaptation. Global transcriptome analyses suggested that more than 100 genes might be regulated by RpoS, but the main part of the regulon remains unexplored. Here, we showed that the expression of bb0689, a gene encoding an outer surface lipoprotein with unknown function, was activated by RpoS. By analyzing gene expression using luciferase reporter assays and quantitative reverse transcription PCR, we found that expression of bb0689 was induced by an elevated temperature, a reduced pH, and increased cell density during in vitro cultivation. The transcriptional start site and a functional promoter for gene expression were identified in the 5' regulatory region of bb0689. The promoter was responsive to environmental stimuli and influenced by RpoS. We also showed that bb0689 expression was expressed in B. burgdorferi during animal infection, suggesting the importance of this gene for infection. We further generated a bb0689 mutant and found that the infectivity of the mutant was severely attenuated in a murine infection model. Although bb0689-deficient spirochetes exhibited no defect during in vitro growth, they were defective in resistance to osmotic stress. Cis-complementation of the mutant with a wild-type copy of bb0689 fully rescued all phenotypes. Collectively, these results demonstrate that the RpoS-regulated gene bb0689 is a key contributor to the optimal infection of B. burgdorferi in animals.