Regulation of FlbD activity by flagellum assembly is accomplished through direct interaction with the trans-acting...

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.