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Journal of Bacteriology, September 2001, p. 5334-5342, Vol. 183, No. 18
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.18.5334-5342.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Mutational Analysis of the Rhizobium lupini H13-3 and Sinorhizobium meliloti Flagellin Genes: Importance of Flagellin A for Flagellar Filament Structure and Transcriptional Regulation

Birgit Scharf,* Henriette Schuster-Wolff-Bühring, Reinhard Rachel, and Rüdiger Schmitt

Institut für Biochemie, Genetik und Mikrobiologie, Universität Regensburg, D-93040 Regensburg, Germany

Received 27 February 2001/Accepted 29 June 2001

Complex flagellar filaments are unusual in their fine structure composed of flagellin dimers, in their right-handed helicity, and in their rigidity, which prevents a switch of handedness. The complex filaments of Rhizobium lupini H13-3 and those of Sinorhizobium meliloti are composed of three and four flagellin (Fla) subunits, respectively. The Fla-encoding genes, named flaA through flaD, are separately transcribed from sigma 28-specific promoters. Mutational analysis of the fla genes revealed that, in both species, FlaA is the principal flagellin and that FlaB, FlaC, and FlaD are secondary. FlaA and at least one secondary Fla protein are required for assembling a functional flagellar filament. Western analysis revealed a ratio close to 1 of FlaA to the secondary Fla proteins (= FlaX) present in wild-type extracts, suggesting that the complex filament is assembled from FlaA-FlaX heterodimers. Whenever a given mutant combination of Fla prevented the assemblage of an intact filament, the biosynthesis of flagellin decreased dramatically. As shown in S. meliloti by reporter gene analysis, it is the transcription of flaA, but not of flaB, flaC, or flaD, that was down-regulated by such abortive combinations of Fla proteins. This autoregulation of flaA is unusual. We propose that any combination of Fla subunits incapable of assembling an intact filament jams the flagellar export channel and thus prevents the escape of an (as yet unidentified) anti-sigma 28 factor that antagonizes the sigma 28-dependent transcription of flaA.

* Corresponding author. Mailing address: Lehrstuhl für Genetik, University of Regensburg, D-93040 Regensburg, Germany. Phone: 49(941)9433170. Fax: 49(941)9433163. E-mail: birgit.scharf{at}biologie.uni-regensburg.de.

Journal of Bacteriology, September 2001, p. 5334-5342, Vol. 183, No. 18
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.18.5334-5342.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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