Characterization of SisK of SisR, an NtrB/C- type regulatory system in the lyme disease spirochete, Borrelia burgdorferi

Lyme disease, an inflammatory disease caused by the spirochete Borrelia burgdorferi, is transmitted by the Ixodes tick. B. burgdorferi survives in prolonged stationary phase in the tick midgut and factors enabling B. burgdorferi to survive such conditions were investigated here. RpoS (S) is an alternative sigma factor in B. burgdorferi that is associated with stress responses and stationary phase survival in other bacteria. Transcription of rpoS in B. burgdorferi is initiated by 54-RNA polymerase holoenzyme and requires the activator SisR. Purified SisR-His binds weakly to a DNA fragment located about 1.6 kb downstream of the rpoS promoter. SisK, the cognate sensor kinase of SisR, was purified as a maltose-binding protein fusion protein (MBP-SisK) and was shown to phosphorylate itself then transfer the phosphate to SisR. SisK contains a PAS domain that may bind flavin since purified MBP-SisK was faint yellow and displayed a peak absorbance at about 410 nm.