Original Research ARTICLE
The RpoS gatekeeper in Borrelia burgdorferi: an invariant regulatory scheme that promotes spirochete persistence in reservoir hosts and niche diversity
- 1University of Connecticut Health Center, United States
- 2Yale University, United States
- 3Connecticut Children's Medical Center, United States
- 4University of Arkansas for Medical Sciences, United States
Maintenance of Borrelia burgdorferi within its enzootic cycle requires a complex regulatory pathway involving the alternative 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 factor, transcribes genes required for tick transmission and mammalian infection. RpoS also exerts a ‘gatekeeper’ function by repressing 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 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 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 regulon 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.
Keywords: Borrelia burgdoferi, Lyme Disease, rpoS, Persistence, Gene regulaiton, vector borne disease, RNAseq analysis, OspA, Sigma Factor, Host Adaptation
Received: 06 Jun 2019;
Accepted: 05 Aug 2019.
Edited by:Axel Cloeckaert, Institut National de la Recherche Agronomique (INRA), France
Reviewed by:Jon Skare, Texas A&M Health Science Center, United States
Jenny A. Hyde, Texas A&M Health Science Center, United States
D. Scott Samuels, University of Montana, United States
Zhiming Ouyang, University of South Florida, United States
Copyright: © 2019 Caimano, Groshong, Belperron, Mao, Hawley, Luthra, Graham, Bockenstedt, Blevins and Radolf. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
* Correspondence: Dr. Melissa J. Caimano, University of Connecticut Health Center, Farmington, United States, email@example.com