title: Regulation of FlbD activity by flagellum assembly is accomplished through direct interaction with the trans-acting factor, FliX.
abstract: The temporal and spatial transcription of late flagellar genes in Caulobacter crescentus is regulated by the sigma54 transcriptional activator, FlbD. One requirement for FlbD activity is the assembly of a structure encoded by early, class II flagellar genes. In this report, we show that the trans-acting factor FliX predominantly functions as a negative regulator of FlbD activity in the absence of the class II-encoded flagellar structure. In contrast, a mutant FliX that bypasses the transcriptional requirement for early flagellar assembly is incapable of repressing FlbD in a class II flagellar mutant. Expression of this mutant allele, fliX1, does not alter the temporal pattern of FlbD-dependent transcription. Remarkably, this mutation confers the correct cell cycle timing of hook operon transcription in a strain that cannot assemble the flagellum, indicating that the progression of flagellar assembly is a minor influence on temporal gene expression. Using a two-hybrid assay, we present evidence that FliX regulates FlbD through a direct interaction, a novel mechanism for this class of sigma54 transcriptional activator. Furthermore, increasing the cellular levels of FliX results in an increase in the concentration of FlbD, and a corresponding increase in FlbD-activated transcription, suggesting that FliX and FlbD form a stable complex in Caulobacter. FliX and FlbD homologues are present in several polar-flagellated bacteria, indicating that these proteins constitute an evolutionarily conserved regulatory pair in organisms where flagellar biogenesis is likely to be under control of the cell division cycle.
authors.affiliation: Department of Chemistry and Biochemistry, Molecular Biology Institute, University of California, Los Angeles, CA 90095-1569, USA.
authors.forename: Rachel E
authors.initials: RE
authors.lastname: Muir
authors.forename: James W
authors.initials: JW
authors.lastname: Gober
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D016935:Caulobacter crescentus / Q000166:cytology / Q000235:genetics / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000378:metabolism*; D019281:Dimerization; D005407:Flagella / Q000502:physiology* / Q000648:ultrastructure; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D017930:Genes, Reporter; D008565:Membrane Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D009876:Operon; D011401:Promoter Regions, Genetic; D016415:Sequence Alignment; D014158:Transcription, Genetic; D020798:Two-Hybrid System Techniques
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; C089631:FlbD protein, Caulobacter crescentus; C111943:FliX protein, Caulobacter crescentus; D008565:Membrane Proteins