Coupling of Flagellar Gene Expression with Assembly in Salmonella Enterica

There are more than 70 genes in the flagellar and chemosensory regulon of Salmonella enterica. These genes are organized into a transcriptional hierarchy of three promoter classes. At the top of the transcriptional hierarchy is the flhDC operon, also called the flagellar master operon, which is transcribed from the flagellar class 1 promoter region. The protein products of the flhDC operon form a hetero-multimeric complex, FlhDC, which directs σ RNA polymerase to transcribe from class 2 flagellar promoters. Products of flagellar class 2 transcription are required for the structure and assembly of the hook-basal body (HBB) complex. One of the class 2 flagellar genes, fliA, encodes an alternative sigma transcription factor, σ, which directs transcription from flagellar class 3 promoters. The class 3 promoters direct transcription of gene products needed after HBB completion including the motor force generators, the filament, and the chemosensory genes. Flagellar gene transcription is coupled to assembly at the level of hook-basal body completion. Two key proteins, σ and FliT, play assembly roles prior to HBB completion and upon HBB completion act as positive and negative regulators, respectively. HBB completion signals a secretion-specificity switch in the flagellar type III secretion system, which results in the secretion of σ and FliT antigonists allowing these proteins to perform their roles in transcriptional regulation of flagellar genes. Genetic methods have provided the principle driving forces in our understanding of how flagellar gene expression is controlled and coupled to the assembly process.