Asymmetrical flagellar rotation in Borrelia burgdorferi nonchemotactic mutants

The Lyme disease spirochete Borrelia burgdorferi has bundles of periplasmic flagella subpolarly located at each cell end. These bundles rotate in opposite directions during translational motility. When not translating, they rotate in the same direction, and the cells flex. Here, we present evidence that asymmetrical rotation of the bundles during translation does not depend upon the chemotaxis signal transduction system. The histidine kinase CheA is known to be an essential component in the signaling pathway for bacterial chemotaxis. Mutants of cheA in flagellated bacteria continually rotate their flagella in one direction. B. burgdorferi has two copies of cheA designated cheA1 and cheA2. Both genes were found to be expressed in growing cells. We reasoned that if chemotaxis were essential for asymmetrical rotation of the flagellar bundles, and if the flagellar motors at both cell ends were identical, inactivation of the two cheA genes should result in cells that constantly flex. To test this hypothesis, the signaling pathway was completely blocked by constructing the double mutant cheA1kan cheA2ermC. This double mutant was deficient in chemotaxis. Rather than flexing, it failed to reverse, and it continually translated only in one direction. Video microscopy of mutant cells indicated that both bundles actively rotated. The results indicate that asymmetrical rotation of the flagellar bundles of spirochetes does not depend upon the chemotaxis system but rather upon differences between the two flagellar bundles. We propose that certain factors within the spirochete localize at the flagellar motors at one end of the cell to effect this asymmetry.