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Role of Integration Host Factor in the Transcriptional Activation of Flagellar Gene Expression in Caulobacter crescentus

Rachel E. Muir^1, and James W. Gober^1*

Department of Chemistry and Biochemistry and Molecular Biology Institute, University of California, Los Angeles, Los Angeles, California¹

Received 14 September 2004/ Accepted 21 October 2004

In the Caulobacter crescentus predivisional cell, class III and IV flagellar genes, encoding the extracytoplasmic components of the flagellum, are transcribed in the nascent swarmer compartment. This asymmetric expression pattern is attributable to the compartmentalized activity of the ⁵⁴-dependent transcriptional activator FlbD. Additionally, these temporally transcribed flagellar promoters possess a consensus sequence for the DNA-binding protein integration host factor (IHF), located between the upstream FlbD binding site and the promoter sequences. Here, we deleted the C. crescentus gene encoding the ß-subunit of the IHF, ihfB (himD), and examined the effect on flagellar gene expression. The ihfB strain exhibited a mild defect in cell morphology and impaired motility. Using flagellar promoter reporter fusions, we observed that expression levels of a subset of class III flagellar promoters were decreased by the loss of IHF. However, one of these promoters, fliK-lacZ, exhibited a wild-type cell cycle-regulated pattern of expression in the absence of IHF. Thus, IHF is required for maximal transcription of several late flagellar genes. The ihfB strain was found to express significantly reduced amounts of the class IV flagellin, FljL, as a consequence of reduced transcriptional activity. Our results indicate that the motility defect exhibited by the ihfB strain is most likely attributable to its failure to accumulate the class IV-encoded 27-kDa flagellin subunit, FljL.

Present address: Department of Genetics, Stanford University, Stanford, CA 94305.