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Journal of Bacteriology, November 2001, p. 6404-6412, Vol. 183, No. 21
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.21.6404-6412.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Structures of Bacterial Flagellar Motors from Two FliF-FliG Gene Fusion Mutants

Dennis Thomas,dagger David Gene Morgan,Dagger and David J. DeRosier*

W. M. Keck Institute for Cellular Visualization, Rosenstiel Basic Medical Sciences Research Center, and Department of Biology, Brandeis University, Waltham, Massachusetts 02454

Received 31 January 2001/Accepted 2 August 2001

Flagella purified from Salmonella enterica serovar Typhimurium contain FliG, FliM, and FliN, cytoplasmic proteins that are important in torque generation and switching, and FliF, a transmembrane structural protein. The motor portion of the flagellum (the basal body complex) has a cytoplasmic C ring and a transmembrane M ring. Incubation of purified basal bodies at pH 4.5 removed FliM and FliN but not FliG or FliF. These basal bodies lacked C rings but had intact M rings, suggesting that FliM and FliN are part of the C ring but not a detectable part of the M ring. Incubation of basal bodies at pH 2.5 removed FliG, FliM, and FliN but not FliF. These basal bodies lacked the C ring, and the cytoplasmic face of the M ring was altered, suggesting that FliG makes up at least part of the cytoplasmic face of the M ring. Further insights into FliG were obtained from cells expressing a fusion protein of FliF and FliG. Flagella from these mutants still rotated but cells were not chemotactic. One mutant is a full-length fusion of FliF and FliG; the second mutant has a deletion lacking the last 56 residues of FliF and the first 94 residues of FliG. In the former, C rings appeared complete, but a portion of the M ring was shifted to higher radius. The C-ring-M-ring interaction appeared to be altered. In basal bodies with the fusion-deletion protein, the C ring was smaller in diameter, and one of its domains occupied space vacated by missing portions of FliF and FliG.

* Corresponding author. Mailing address: Rosenstiel Center MS029, Brandeis University, Waltham, MA 02454. Phone: (781) 736-2494. Fax: (781) 736-2405. E-mail: derosier{at}brandeis.edu.

dagger Present address: Structural Biology Program, European Molecular Biology Laboratory, Heidelberg, Germany.

Dagger Present address: Department of Biophysics, Boston University School of Medicine, Boston, MA 02118.

Journal of Bacteriology, November 2001, p. 6404-6412, Vol. 183, No. 21
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.21.6404-6412.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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