HilA , is believed to directly activate expression from the prgH promoters in SPI1 .
These results suggest that repression of hilA transcription by oxygen leads to a decrease in HilA protein production , which in turn reduces the expression of HilA-activated genes , such as prgH .
HilA directly activates the prgH promoter .
By binding upstream of prgH , HilA directly activates expression of the prgH operons that encode the components of the TTS apparatus .