Borrelia burgdorferi uniquely regulates its motility genes and has an intricate flagellar hook basal body structure

Department of Microbiology, Immunology, and Cell Biology, Robert C. Byrd Health Sciences Center, West Virginia University, Morgantown, West Virginia 26506-9177; Department of Oral Biology, State University of New York at Buffalo, Buffalo, NY 14214; Prefectural University of Hiroshima, Department of Life Sciences, 562 Nanatsuka, Shobara, Hiroshima, 727-0 023, Japan and CREST "Soft-Nano machine Project, Innovation Plaza, Hiroshima,3-10-23 Kagamiyama, Higashi-Hiroshima, 739-0046, Japan

^* To whom correspondence should be addressed. Email: ncharon{at}hsc.wvu.edu.

	Abstract

Borrelia burgdorferi is a flat-wave, motile spirochete that causes Lyme disease. Motility is provided by periplasmic flagella (PFs) located between the cell cylinder and an outer membrane sheath. These PFs, composed of basal body, hook, and filament, are similar in structure to flagella from other bacteria. To determine if hook formation influences flagellin gene transcription in B. burgdorferi, we inactivated the hook structural gene flgE by targeted mutagenesis. In many bacteria, completion of the hook structure serves as a checkpoint for transcriptional control of flagella synthesis and other chemotaxis and motility genes. Specifically, the hook allows secretion of the anti-sigma factor FlgM, and concomitant late genes transcription promoted by ²⁸. However, the control of B. burgdorferi PF synthesis differs from flagella synthesis in other bacteria: The gene encoding ²⁸ is not present in its genome, nor are there any ²⁸ promoter recognition sequences associated with its motility genes. We found that B. burgdorferi mutants in flgE lacked PFs, were rod-shaped, and were non-motile, which substantiate previous evidence that PFs are involved in both cell morphology and motility. Although most motility and chemotaxis gene products accumulated at wild-type levels in the absence of FlgE, mutant cells had markedly decreased levels of flagellar filament proteins FlaA and FlaB. Further analyses showed that reduction of flagellin proteins in the spirochetes lacking FlgE was mediated at the post-transcriptional level. Taken together, our results indicate that in B. burgdorferi, the completion of the hook does not serve as a checkpoint for transcriptional regulation of flagella synthesis. In addition, we also present evidence that the hook protein in B. burgdorferi forms a high molecular weight complex, and that formation of this complex occurs in the periplasmic space.