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The intracellular pathogen Salmonella enterica serovar Typhimurium (S. Typhimurium) exploits host macrophage as a crucial survival and replicative niche. To minimize host immune response stimulated by flagellin, the expression of flagellar genes is downregulated during S. Typhimurium growth within host macrophages. However, the underlying mechanisms are largely unknown. In this study, we show that STM14_1285 (named AsiR), a putative RpiR-family transcriptional regulator, which is downregulated within macrophages as previously reported and also confirmed here, positively regulates the expression of flagellar genes by directly binding to the promoter of flhDC. By generating an asiR mutant strain and a strain that persistently expresses asiR gene within macrophages, we confirmed that the downregulation of asiR contributes positively to S. Typhimurium replication in macrophages and systemic infection in mice, which could be attributed to decreased flagellar gene expression and therefore reduced flagellin-stimulated secretion of pro-inflammatory cytokines IL-1β and TNF-α. Furthermore, the acidic pH in macrophages is identified as a signal for the downregulation of asiR and therefore flagellar genes. Collectively, our results reveal a novel acidic pH signal-mediated regulatory pathway that is utilized by S. Typhimurium to promote intracellular replication and systemic pathogenesis by repressing flagellar gene expression.

Enterohemorrhagic Escherichia coli (EHEC) O157:H7 is a human pathogen that causes a variety of diseases, such as hemorrhagic colitis and lethal hemolytic uremic syndrome. Flagellum-dependent motility plays diverse roles in the pathogenesis of EHEC O157:H7, including its migration to an optimal host site, adherence and colonization, survival at the infection site, and post-infection dispersal. However, it is very expensive for cellular economy in terms of the number of genes and the energy required for flagellar biosynthesis and functioning. Furthermore, the flagellar filament bears strong antigenic properties that induce a strong host immune response. Consequently, the flagellar gene expression and biosynthesis are highly regulated to occur at the appropriate time and place by different regulatory influences. The present review focuses on the regulatory mechanisms of EHEC O157:H7 motility and flagellar biosynthesis, especially in terms of flagellar gene regulation by environmental factors, regulatory proteins, and small regulatory RNAs.

The intracellular pathogen Salmonella enterica serovar Typhimurium (S. Typhimurium) exploits host macrophage as a crucial survival and replicative niche. To minimize host immune response stimulated by flagellin, the expression of flagellar genes is downregulated during S. Typhimurium growth within host macrophages. However, the underlying mechanisms are largely unknown. In this study, we show that STM14_1285 (named AsiR), a putative RpiR-family transcriptional regulator, which is downregulated within macrophages as previously reported and also confirmed here, positively regulates the expression of flagellar genes by directly binding to the promoter of flhDC. By generating an asiR mutant strain and a strain that persistently expresses asiR gene within macrophages, we confirmed that the downregulation of asiR contributes positively to S. Typhimurium replication in macrophages and systemic infection in mice, which could be attributed to decreased flagellar gene expression and therefore reduced flagellin-stimulated secretion of pro-inflammatory cytokines IL-1β and TNF-α. Furthermore, the acidic pH in macrophages is identified as a signal for the downregulation of asiR and therefore flagellar genes. Collectively, our results reveal a novel acidic pH signal-mediated regulatory pathway that is utilized by S. Typhimurium to promote intracellular replication and systemic pathogenesis by repressing flagellar gene expression. [ABSTRACT FROM AUTHOR]

For self-assembly of the bacterial flagellum, most of the flagellar component proteins synthesized in the cytoplasm are exported by the flagellar type Ill export apparatus to the growing, distal end. Flagellar protein export is highly organized and well controlled in every step of the flageilar assembly process. Flagellar-specific chaperones not only facilitate the export of their cognate proteins, as well as prevent their premature aggregation in the cytoplasm, but also play a role in fine- tuning flagellar gene expression to be coupled with the flagellar assembly process. FliT is a flagellar-specific chaperone responsible for the export of the filament-cappWig protein FliD and for negative control of flagellar gene expression by binding to the FIhDC complex. Here we report the crystal structure of Salmonella FliT at 3.2-A resolution. The structural and biochemical analyses clearly reveal that the C-terminal segment of FliT regulates its interactions with the FIhDC complex, Flil ATPase, and Fiji (subunits of the export apparatus), and that its conformational change is responsible for the switch in its binding partners during flagellar protein export. [ABSTRACT FROM AUTHOR]

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, FlhD 4 C 2 , which directs σ 70 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 28 , 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 28 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 28 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.