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

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.