Continuous control of flagellar gene expression by the σ28–FlgM regulatory circuit in Salmonella enterica

Summary The flagellar genes in Salmonella enterica are expressed in a temporal hierarchy that mirrors the assembly process itself. The σ28–FlgM regulatory circuit plays a key role in controlling this temporal hierarchy. This circuit ensures that the class 3 genes are expressed only when the hook‐basal body (HBB), a key intermediate in flagellar assembly, is complete. In this work, we investigated the role of the σ28–FlgM regulatory circuit in controlling the timing and magnitude of class 3 gene expression using a combination of mathematical modelling and experimental analysis. Analysis of the model predicted that this circuit continuously controls class 3 gene expression in response to HBB abundance. We experimentally validated these predictions by eliminating different components of the σ28–FlgM regulatory system and also by rewiring the transcriptional hierarchy. Based on these results, we conclude that the σ28–FlgM regulatory circuit continuously senses the HBB assembly process and regulates class 3 gene expression and possibly flagellar numbers in response.