HrpA, an RNA Helicase Involved in RNA Processing, Is Required for Mouse Infectivity and Tick Transmission of the Lyme Disease Spirochete

Abstract

The Lyme disease spirochete Borrelia burgdorferi must differentially express genes and proteins in order to survive in and transit between its tick vector and vertebrate reservoir. The putative DEAH-box RNA helicase, HrpA, has been recently identified as an addition to the spirochete's global regulatory machinery; using proteomic methods, we demonstrated that HrpA modulates the expression of at least 180 proteins. Although most bacteria encode an HrpA helicase, RNA helicase activity has never been demonstrated for HrpAs and the literature contains little information on the contribution of this protein to bacterial physiology or pathogenicity. In this work, we report that B. burgdorferi HrpA has RNA-stimulated ATPase activity and RNA helicase activity and that this enzyme is essential for both mammalian infectivity by syringe inoculation and tick transmission. Reduced infectivity of strains carrying mutations in the ATPase and RNA binding motif mutants suggests that full virulence expression requires both ATPase and coupled helicase activity. Microarray profiling revealed changes in RNA levels of two-fold, or less in an hrpA mutant versus wild-type, suggesting that the enzyme functions largely or exclusively at the post-transcriptional level. In this regard, northern blot analysis of selected gene products highly regulated by HrpA ( bb0603 [ p66], bba74, bb0241 [ glpK], bb0242 and bb0243 [ glpA]) suggests a role for HrpA in the processing and translation of transcripts. In addition to being the first demonstration of RNA helicase activity for a bacterial HrpA, our data indicate that the post-transcriptional regulatory functions of this enzyme are essential for maintenance of the Lyme disease spirochete's enzootic cycle.

Author Summary

The bacterium causing Lyme disease, Borrelia burgdorferi, must differentially express genes and proteins in order to survive in and transit between its tick vector and animals that it infects. RNA helicases, enzymes that unwind double-stranded RNA, have recently emerged as major players in all types of processes involving RNA in higher organisms. But in spite of the ubiquitous presence of RNA helicases in bacteria, little is known regarding their function. The Lyme disease spirochete, which has a complex lifecycle involving ticks and vertebrate animals, encodes a single putative RNA helicase, HrpA. Here we establish that the purified protein indeed displays both ATPase and RNA helicase activity. In addition to being the first demonstration of RNA helicase activity for a bacterial HrpA, our data indicate that HrpA is involved in RNA processing of some genes. Our findings also show that HrpA is essential for both tick transmission and mouse infection and establish the RNA helicase as an important component in both parts of the spirochete's lifecycle.