Roles of hilC and hilD in regulation of hilA expression in Salmonella enterica serovar typhimurium

Sequences between -332 and -39 upstream of the hilA promoter are required f or repression of hilA, An unidentified repressor is thought to bind these u pstream repressing sequences (URS) to inhibit hilA expression, Two AraC-lik e transcriptional regulators encoded on Salmonella pathogenicity island 1 ( SPI1), HilC and HilD, bind to the URS to counteract the repression of hilA. The URS is required for regulation of hilA by osmolarity, oxygen, PhoP/Pho Q, and SirA/BarA. Here, we show that FadD, FliZ, PhoB, and EnvZ/OmpR also r equire the URS to regulate hilA. These environmental and regulatory factors may affect hilA expression by altering the expression or activity of HilC, HilD, or the unknown repressor, To begin investigating these possibilities , we tested the effects of environmental and regulatory factors on hilC and hilD expression. We also examined hilA regulation when hilC or hilD was di srupted or expressed to a high level. Although hilC is regulated by all env ironmental conditions and regulatory factors that modulate hilA expression, hilC is not required for the regulation of hilA by any conditions or facto rs except EnvZ/OmpR. In contrast, hilD is absolutely required for hilA expr ession, but environmental conditions and regulatory factors have little or no effect on hilD expression, We speculate that EnvZ/OmpR regulates hilA by altering the expression and/or activity of hilC, while all other regulator y conditions and mutations regulate hilA by modulating hilD posttranscripti onally, We also discuss models in which the regulation of hilA expression i s mediated by modulation of the expression or activity of one or more repre ssors.