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Journal of Bacteriology, October 1999, p. 6160-6170, Vol. 181, No. 19
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

FlbT Couples Flagellum Assembly to Gene Expression in Caulobacter crescentus

Erin K. Mangan,1,dagger Jaleh Malakooti,2 Anthony Caballero,2 Paul Anderson,1 Bert Ely,2 and James W. Gober1,*

Department of Chemistry and Biochemistry and Molecular Biology Institute, University of California---Los Angeles, Los Angeles, California 90095-1569,1 and Department of Biological Sciences, University of South Carolina, Columbia, South Carolina 292082

Received 27 April 1999/Accepted 22 July 1999

The biogenesis of the polar flagellum of Caulobacter crescentus is regulated by the cell cycle as well as by a trans-acting regulatory hierarchy that functions to couple flagellum assembly to gene expression. The assembly of early flagellar structures (MS ring, switch, and flagellum-specific secretory system) is required for the transcription of class III genes, which encode the remainder of the basal body and the external hook structure. Similarly, the assembly of class III gene-encoded structures is required for the expression of the class IV flagellins, which are incorporated into the flagellar filament. Here, we demonstrate that mutations in flbT, a flagellar gene of unknown function, can restore flagellin protein synthesis and the expression of fljK::lacZ (25-kDa flagellin) protein fusions in class III flagellar mutants. These results suggest that FlbT functions to negatively regulate flagellin expression in the absence of flagellum assembly. Deletion analysis shows that sequences within the 5' untranslated region of the fljK transcript are sufficient for FlbT regulation. To determine the mechanism of FlbT-mediated regulation, we assayed the stability of fljK mRNA. The half-life (t1/2) of fljK mRNA in wild-type cells was approximately 11 min and was reduced to less than 1.5 min in a flgE (hook) mutant. A flgE flbT double mutant exhibited an mRNA t1/2 of greater than 30 min. This suggests that the primary effect of FlbT regulation is an increased turnover of flagellin mRNA. The increased t1/2 of fljK mRNA in a flbT mutant has consequences for the temporal expression of fljK. In contrast to the case for wild-type cells, fljK::lacZ protein fusions in the mutant are expressed almost continuously throughout the C. crescentus cell cycle, suggesting that coupling of flagellin gene expression to assembly has a critical influence on regulating cell cycle expression.

* Corresponding author. Mailing address: Department of Chemistry and Biochemistry and Molecular Biology Institute, University of California---Los Angeles, Los Angeles, CA 90095-1569. Phone: (310) 206-9449. Fax: (310) 206-5213. E-mail: gober{at}chem.ucla.edu.

dagger Present address: University of Alabama at Birmingham, Department of Medicine, Division of Clinical Immunology and Rheumatology, Birmingham, AL 35294-0006.

Journal of Bacteriology, October 1999, p. 6160-6170, Vol. 181, No. 19
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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