The Salmonella Spi1 Virulence Regulatory Protein HilD Directly Activates Transcription of the Flagellar Master Operon flhDC

Infection of intestinal epithelial cells is dependent on the Salmonella enterica serovar Typhimurium pathogenicity island 1 (Spi1)-encoded type III injectisome system and flagellar motility. Thus, the expression of virulence and flagellar genes is subject to tight regulatory control mechanisms in order to ensure the correct spatiotemporal production of the respective gene products. In this work, we reveal a new level of cross-regulation between the Spi1 and flagellar regulatory systems. Transposonmutagene-sis identified a class of mutants that prevented flhDC autorepression by overexpressing HilD. HilD, HilC, RtsA, and HilA com-prise a positive regulatory circuit for the expression of the Spi1 genes. Here, we report a novel transcriptional cross talk between the Spi1 and flagellar regulons where HilD transcriptionally activates flhDC gene expression by binding to nucleotides68 to 24 upstream from the P5 transcriptional start site. We additionally show that, in contrast to the results of a previous report, HilA does not affect flagellar gene expression. Finally, we discuss a model of the cross-regulation network between Spi1 and the flagellar system and propose a regulatory mechanism via the Spi1 master regulator HilD that would prime flagellar genes for rapid reactivation during host infection. The enteropathogenic Gram-negative bacteria of the genus Sal-monella are responsible for the food-borne illness gastroen-teritis, localized infection of the small intestine, and systemic en-teric (typhoid) fevers. Symptoms of Salmonella infection include diarrhea, abdominal cramps, and fever (1). The effector-drive