BosR (BB0647) governs virulence expression in Borrelia burgdorferi

Abstract

SummaryBorrelia burgdorferi (Bb), the Lyme disease spirochaete, encodes a potential ferric uptake regulator (Fur) homologue, BosR (BB0647). Thus far, a role for BosR in Bb metabolism, gene regulation or pathogenesis has not been determined, largely due to the heretofore inability to inactivate bosR in low-passage, infectious Bb isolates. Herein, we report the generation of the first bosR-deficient mutant in a virulent strain of Bb. Whereas the bosR mutant persisted normally in ticks, the mutant was unable to infect mice, indicating that BosR is essential for Bb infection of a mammalian host. Moreover, transcriptional profiling of the bosR mutant showed that a number of genes were either positively or negatively influenced by BosR deficiency, suggesting that BosR may function both as a global repressor and activator in Bb. Strikingly, our study showed that BosR controls the expression of two major virulence-associated Bb lipoproteins, OspC and DbpA, likely via an influence on the alternative sigma factor, RpoS. This study thus not only has elucidated another key virulence gene of Bb, but also provides new insights into a previously unknown layer of gene regulation governing RpoS in Bb.

The Fur homologue BosR requires Arg39 to activate rpoS transcription in Borrelia burgdorferi and thereby direct spirochaete infection in mice

“…Others have reported that bosR-knockouts of B. burgdorferi are not infectious in mice (Hyde et al, 2009;Ouyang et al, 2009Ouyang et al, , 2011. Here, the WT strain B31-A3 was infectious at a dose of 20 000 spirochaetes per mouse, whereas neither of the bosR-mutant strains (L9 and K18) was infectious at this dose or at a higher dose of 100 000 spirochaetes per mouse (Table 1 and data not shown).…”

“…Strains B31-MI and 297 are infectious in mice (Ouyang et al, 2009(Ouyang et al, , 2011; however, strain ML23 lacks the lp25 linear plasmid and therefore is not infectious in mice (Hyde et al, 2009). Here, strain B31-A3 was chosen for study.…”

“…In previous studies, knockouts of bosR were generated in strains B31-MI (Ouyang et al, 2009), ML23 (Hyde et al, 2009) and 297 (Ouyang et al, 2011). Strains B31-MI and 297 are infectious in mice (Ouyang et al, 2009(Ouyang et al, , 2011; however, strain ML23 lacks the lp25 linear plasmid and therefore is not infectious in mice (Hyde et al, 2009).…”

“…Production of the short rpoS transcript is tightly regulated, requiring both the alternative sigma factor RpoN (Hübner et al, 2001) and the Fur homologue BosR (Hyde et al, 2009;Ouyang et al, 2009). To initiate transcription, RpoN must first be activated by the response regulator Rrp2 (Yang et al, 2003) which, in turn, must first be phosphorylated (Xu et al, 2010).…”

“…Although BosR may be involved in the oxidative stress response (Hyde et al, 2009(Hyde et al, , 2010), it appears its main function is to act as a global regulator. To prepare the spirochaete for mammalian infection, BosR upregulates transcription of rpoS (Hyde et al, 2009(Hyde et al, , 2010Ouyang et al, 2009Ouyang et al, , 2011 and also downregulates transcription of various tick-phase genes (e.g. ospA and ospD), which need only be expressed in the tick and which if allowed expression in the mammal could lead to immune clearance (Shi et al, 2014;Wang et al, 2013).…”