The role of carbon storage regulator A (CsrABb) in the patho-physiology of Borrelia burgdorferi

Borrelia burgdorferi, the agent of Lyme disease, alters its gene expression in response to highly disparate environmental signals encountered in its hosts. Among the relatively few regulators of adaptive gene expression present in the borrelial genome is an open reading frame (ORF), bb0184, annotated as CsrA (carbon storage regulator A). CsrA, in several bacterial species, has been characterized as a small RNA binding protein that functions as a global regulator affecting mRNA stability or the levels of translation of multiple ORFs. CsrA in Borrelia burgdorferi (CsrABb ), is upregulated in response to propagation of B. burgdorferi under mammalian host-mimicking conditions. In order to further delineate the role of CsrABb, we generated a deletion mutant of CsrA Bb. Analysis of the mutant that was propagated under conditions mimicking the vertebrate host (pH 6.8/37°C ) revealed a significant reduction in the levels of vertebrate host specific determinants compared to the parental wild-type strain including two regulatory proteins, RpoS and BosR. In Borrelia burgdorferi, CsrABb post-transcriptionally represses the Phosphate acetyl transferase (Pta), enzyme which converts acetyl phosphate to acetyl-CoA leading to accumulation of Acetyl phosphate. Acetyl phosphate activates the Rrp2-RpoN-RpoS pathway and facilitates the vertebrate host-specific adaptation and is critical for infectivity. Increased levels of supplemental acetate restored vertebrate host-responsive determinants in the csrA Bb mutant to parental levels indicating that both the levels CsrABb and acetyl phosphate/acetyl-CoA balance contribute to the activation of the Rrp2-RpoN-RpoS pathway. Site-specific replacement of 8 key residues of CsrABb (8S) with alanines resulted in increased levels of CsrABb and reduced levels of Pta with a concomitant increase in levels of RpoS, BosR, and other members of rpoS regulon. Truncation of 7 amino acids at the C-terminus of CsrABb (7D) resulted in reduced CsrABb transcripts and post-transcriptionally reduced levels of FliW located upstream of CsrABb. These observations underscore that CsrABb has conserved and unique functions consistent with the patho-physiology of B. burgdorferi.