Multiple factors independently regulate hilA and invasion gene expression in Salmonella enterica serovar typhimurium

HilA activates the expression of Salmonella enterica serovar Typhimurium in vasion genes. To learn more about regulation of hilA, we isolated Tn5 mutan ts exhibiting reduced hilA and/or invasion gene expression. In addition to expected mutations, we identified Tn5 insertions in pstS, fadD, fihD, fihC, and fliA. Analysis of the pstS mutant indicates that hilA and invasion gen es are repressed by the response regulator PhoB in the absence of the Pst h igh-affinity inorganic phosphate uptake system. This system is required for negative control of the PhoR-PhoB two-component regulatory system, suggest ing that hilA expression may be repressed: by PhoR-PhoB under low extracell ular inorganic phosphate conditions. FadD is required for uptake and degrad ation of long-chain fatty acids, and our analysis of the fadD mutant indica tes that hilA is regulated by a FadD-dependent, FadR-independent mechanism. Thus, fatty acid derivatives may act as intracellular signals to regulate hilA expression, fihDC and fliA encode transcription factors required for f lagellum production, motility, and chemotaxis. Complementation studies with flhC and fliA mutants indicate that FliZ which is encoded in an operon wit h fliA, activates expression of hilA, linking regulation of hilA with motil ity. Finally, epistasis tests showed that PhoB, FadD, FliZ, SirA, and EnvZ act independently to regulate hilA expression and invasion. In summary, our screen has identified several distinct pathways that can modulate S. enter ica serovar Typhimurium's ability to express hilA and invade host cells. In tegration of signals from these different pathways may help restrict invasi on gene expression during infection.