A small RNA regulates RpoS in the Lyme disease spirochete Borrelia burgdorferi

We have identified and characterized the first sRNA, DsrABb, in the Lyme disease spirochete, Borrelia burgdorferi; as well as, identified a non-canonical RNA chaperone Hfq. The alternative sigma factor RpoS (sigma 38 or sigma S) plays a central role in the reciprocal regulation of the virulence-associated major outer surface proteins OspC and OspA. Temperature is one of the key environmental signals controlling RpoS, but the molecular mechanism by which the signal is transduced remains unknown. DsrABb post-transcriptionally regulates the alternative sigma factor RpoS in response to an increase in temperature, associated with the tick to mammal transmission signal. A novel 5\u27 end of the rpoS mRNA was identified and DsrABb has the potential of extensively base-pairing with the upstream region of this rpoS transcript. We demonstrate that B. burgdorferi strains lacking DsrABb do not upregulate RpoS and OspC in response to an increase in temperature, but do regulate RpoS and OspC in response to changes in pH and cell density. The 5\u27 and 3\u27 ends of DsrABb were mapped, demonstrating that at least four species exist with sizes ranging from 213 to 352 nucleotides. We demonstrate and characterize in vitro the interaction between DsrA and upstream region of rpoS mRNA. We hypothesize that DsrABb binds to the upstream region of the rpoS mRNA and stimulates translation by releasing the Shine-Dalgarno sequence and start site from a stable secondary structure. Therefore, we postulate that DsrABb is a molecular thermometer regulating RpoS in Borrelia burgdorferi