DksA-dependent regulation of RpoS contributes toBorrelia burgdorferitick-borne transmission and mammalian infectivity
Authors: Boyle William K. , Richards Crystal L. , Dulebohn Daniel P. , Zalud Amanda K. , Shaw Jeff A. , Lovas Sándor , Gherardini Frank C. , Bourret Travis J. 
Published: November 2020

Throughout its enzootic cycle, the Lyme disease spirocheteBorreliella (Borrelia) burgdorferi, senses and responds to changes in its environment by using a small repertoire of transcription factors which coordinate the expression of genes required for infection ofIxodesticks and various mammalian hosts. Among these transcription factors, the DnaK suppressor protein (DksA) plays a pivotal role in regulating gene expression inB. burgdorferiduring periods of nutrient limitation and is required for mammalian infectivity. In many pathogenic bacteria, the gene regulatory activity of DksA along with the alarmone guanosine penta- and tetra-phosphate ((p)ppGpp) coordinates the stringent response to various environmental stresses including nutrient limitation. In this study, we sought to characterize the role of DksA in regulating the transcriptional activity of RNA polymerase and in the regulation of RpoS-dependent gene expression required forB. burgdorferiinfectivity. Usingin vitrotranscription assays, we observed recombinant DksA inhibits RpoD-dependent transcription byB. burgdorferiRNA polymerase independent of ppGpp Additionally, we determined the pH-inducible expression of RpoS-dependent genes relies on DksA, but is independent of (p)ppGpp produced by Relbbu. Subsequent transcriptomic and western blot assays indicated DksA regulates the expression of BBD18, a protein previously implicated in the post-transcriptional regulation of RpoS. Moreover, we observed DksA was required for infection of mice following intraperitoneal inoculation or for transmission ofB. burgdorferibyIxodes scapularisnymphs. Together, these data suggest DksA plays a central role in coordinating transcriptional responses ofB. burgdorferirequired for infectivity through its interactions with RNA polymerase and post-transcriptional control of RpoS.

Author Summary

Lyme disease, caused by the spirochetal bacteriaBorrelia burgdorferi, is the most common vector-borne illness in North America. The ability ofB. burgdorferito establish infection is predicated by its ability to coordinate the expression of virulence factors in response to diverse environmental stimuli encountered withinIxodesticks and mammalian hosts. Previous studies have shown an essential role for the alternative sigma factor RpoS in regulating the expression of genes required for the successful transmission ofB. burgdorferibyIxodesticks and infection of mammalian hosts. The DnaK suppressor protein (DksA) is a global gene regulator inB. burgdorferithat also contributes to the expression of RpoS-dependent genes. In this study, we determined DksA exerts its gene regulatory function through direct interactions with theB. burgdorferiRNA polymerase usingin vitrotranscription assays and controls the expression of RpoS-dependent genes required for mammalian infection by post-transcriptionally regulating cellular levels of RpoS. Our results demonstrate the utility ofin vitrotranscription assays to determine how gene regulatory proteins like DksA control gene expression inB. burgdorferi, and reveal a novel role for DksA in the infectious cycle ofB. burgdorferi.