Characterization of 6S RNA in the Lyme disease spirochete

Abstract

6S RNA binds to RNA polymerase and regulates gene expression, contributing to bacterial adaptation to environmental stresses. In this study, we examined the role of 6S RNA in murine infectivity and tick persistence of the Lyme disease spirochete Borrelia (Borreliella) burgdorferi. B. burgdorferi 6S RNA (Bb6S RNA) binds to RNA polymerase, is expressed independent of growth phase or nutrient stress in culture, and is processed by RNase Y. We found that rny (bb0504), the gene encoding RNase Y, is essential for B. burgdorferi growth, while ssrS, the gene encoding 6S RNA, is not essential, indicating a broader role for RNase Y activity in the spirochete. Bb6S RNA regulates expression of the ospC and dbpA genes encoding outer surface protein C and decorin binding protein A, respectively, which are lipoproteins important for host infection. The highest levels of Bb6S RNA are found when the spirochete resides in unfed nymphs. ssrS mutants lacking Bb6S RNA were compromised for infectivity by needle inoculation, but injected mice seroconverted, indicating an ability to activate the adaptive immune response. ssrS mutants were successfully acquired by larval ticks and persisted through fed nymphs. Bb6S RNA is one of the first regulatory RNAs identified in B. burgdorferi that controls the expression of lipoproteins involved in host infectivity.

Comparative transcriptomic analysis of Rickettsia conorii during in vitro infection of human and tick host cells

“…In Borrelia , 6S RNA exhibits highest expression levels in Ixodes unfed nymphs compared to fed nymphs and ssrS deletion mutant is compromised in infectivity of mice. Interestingly, despite being seropositive, the number of antigenic proteins reacting with murine immune system are considerably less in the ∆ ssrS mutant compared to wild type or ssrS complemented strain, implicating a role for 6S RNA in the regulation of the expression of genes targeted by the murine adaptive immune system [ 82 ].…”

Gene Regulation and Transcriptomics

“…RelBbu may also indirectly modulate RNA polymerase activity to alter the global transcriptome through the Bb6S RNA (see the section below on Small regulatory RNAs and RNA chaperones). Bb6S RNA is a highly expressed sRNA that binds directly to the RpoD (σ 70 )-RNA polymerase (Drecktrah et al, 2020). RelBbu upregulates Bb6S RNA, potentially providing another mechanism to shape the transcriptional landscape in addition to the direct interaction of (p)ppGpp with RNA polymerase (Drecktrah et al, 2018).…”

“…(i) DsrABb is a trans-acting sRNA that base-pairs with rpoS mRNA regulating its translation (Lybecker and Samuels, 2007). (ii) Bb6S RNA is a proteininteracting sRNA that directly influences σ factor selectivity by RNA polymerase and globally regulates gene expression (Drecktrah et al, 2020). (iii) ittA is an sRNA that affects expression of several genes and is required for dissemination in the vertebrate host via an unknown molecular mechanism (Medina-Pérez et al, 2020).…”

“…Bb6S RNA, the canonical 6S RNA in B. burgdorferi, was first predicted by computational genome mining (Barrick et al, 2005) and was then found in the noncoding sRNA transcriptomes of B. burgdorferi (Arnold et al, 2016;Popitsch et al, 2017;Drecktrah et al, 2018). Recently, Bb6S RNA was shown to bind RNA polymerase in vivo, be processed by RNase Y, regulate expression of lipoprotein genes, and play a role in infectivity in the murine model (Drecktrah et al, 2020). Mapping and modeling of Bb6S RNA suggest that the secondary structure is conserved with a single-stranded central bubble surrounded by double-stranded stem regions (Barrick et al, 2005;Trotochaud and Wassarman, 2005;Chen et al, 2017;Drecktrah et al, 2020).…”

“…Recently, Bb6S RNA was shown to bind RNA polymerase in vivo, be processed by RNase Y, regulate expression of lipoprotein genes, and play a role in infectivity in the murine model (Drecktrah et al, 2020). Mapping and modeling of Bb6S RNA suggest that the secondary structure is conserved with a single-stranded central bubble surrounded by double-stranded stem regions (Barrick et al, 2005;Trotochaud and Wassarman, 2005;Chen et al, 2017;Drecktrah et al, 2020).…”