The RpoS Gatekeeper in Borrelia burgdorferi: An Invariant Regulatory Scheme That Promotes Spirochete Persistence in Reservoir Hosts and Niche Diversity

Abstract

Maintenance of Borrelia burgdorferi within its enzootic cycle requires a complex regulatory pathway involving the alternative σ factors RpoN and RpoS and two ancillary trans -acting factors, BosR and Rrp2. Activation of this pathway occurs within ticks during the nymphal blood meal when RpoS, the effector σ factor, transcribes genes required for tick transmission and mammalian infection. RpoS also exerts a ‘gatekeeper’ function by repressing σ 70 -dependent tick phase genes (e.g., ospA , lp6.6 ). Herein, we undertook a broad examination of RpoS functionality throughout the enzootic cycle, beginning with modeling to confirm that this alternative σ factor is a ‘genuine’ RpoS homolog. Using a novel dual color reporter system, we established at the single spirochete level that ospA is expressed in nymphal midguts throughout transmission and is not downregulated until spirochetes have been transmitted to a naïve host. Although it is well established that rpoS /RpoS is expressed throughout infection, its requirement for persistent infection has not been demonstrated. Plasmid retention studies using a trans -complemented Δ rpoS mutant demonstrated that (i) RpoS is required for maximal fitness throughout the mammalian phase and (ii) RpoS represses tick phase genes until spirochetes are acquired by a naïve vector. By transposon mutant screening, we established that bba34/oppA5 , the only OppA oligopeptide-binding protein controlled by RpoS, is a bona fide persistence gene. Lastly, comparison of the strain 297 and B31 RpoS DMC regulons identified two cohorts of RpoS-regulated genes. The first consists of highly conserved syntenic genes that are similarly regulated by RpoS in both strains and likely required for maintenance of B. burgdorferi sensu stricto strains in the wild. The second includes RpoS-regulated plasmid-encoded variable surface lipoproteins ospC , dbpA and members of the ospE/ospF/elp , mlp , revA , and Pfam54 paralogous gene families, all of which have evolved via inter- and intra-strain recombination. Thus, while the RpoN/RpoS pathway regulates a ‘core’ group of orthologous genes, diversity within RpoS regulons of different strains could be an important determinant of reservoir host range as well as spirochete virulence.

A Novel Rapid Sample Preparation Method for MALDI-TOF MS Permits Borrelia burgdorferi Sensu Lato Species and Isolate Differentiation

“…It is known that Borrelia spp. change the expression patterns of proteins during host or vector phases of their life cycle (Radolf et al, 2012;Caimano et al, 2019) and many of the involved genes are located on plasmids (Casjens et al, 2012(Casjens et al, , 2017. Research into expression patterns of proteins of Borrelia spp.…”

Borrelia bavariensis: Vector Switch, Niche Invasion, and Geographical Spread of a Tick-Borne Bacterial Parasite

“…In addition, studies on the gene expression regulation system RpoS identified 41 genes that are repressed in B. burgdorferi in vitro and under conditions representing the vertebrate host. Although a good number encoded hypothetical proteins, many of the identified genes were known to be important for survival in, and interaction with, the vector (Caimano et al, 2019). Whether variations in these molecules contribute to differential vector competence needs to be further investigated.…”

Role of HK2 in the Enzootic Cycle of Borrelia burgdorferi

“…The Rrp2-RpoN-RpoS pathway, or the σ 54 -σ S alternative sigma factor cascade, is the most studied regulatory pathway in B. burgdorferi (13,14). It plays a major role in controlling differential gene expression during the process of the spirochetal transmission from ticks to mammals and has thus been called "Gatekeeper" (11). Therefore, understanding how this pathway is activated is important for our understanding of how B. burgdorferi migrates between ticks and mammals.…”

“…Comparing to free living bacteria such as Escherichia coli, which has more than 30 two-component signal transduction systems (TCSs), B. burgdorferi has reduced to two sets of TCS, HK1/Rrp1 and HK2/Rrp2 (in addition to the chemotactic CheA-CheY system) and has evolved to employ these two TCSs to survive in each of the two hosts encountered in the enzootic cycle. HK1/Rrp1, a c-di-GMP producing system, controls spirochete's adaptation to the tick vector (1-7), whereas HK2/Rrp2 is essential for B. burgdorferi to establish infection in the mammalian host (8)(9)(10)(11).…”

“…Thus, upon activation by phosphorylation at its N-terminal receiver domain, Rrp2 activates its central domain, leading to activation of an alternative sigma factor cascade, σ 54 -σ S cascade (also called RpoN-RpoS pathway or Rrp2-RpoN-RpoS pathway) in B. burgdorferi [for review, see (13)(14)(15)]. Rrp2, along with a transcriptional activator BosR and a repressor BadR, regulates σ 54 -σ S sigma factor cascade, which in turn controls ospC, dbpB/A, bbk32, and many other mammalian infection-associated genes (8)(9)(10)(11)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25)(26).…”