I think the results of this paper will help you select the right gene.
"We have previously shown that cultured epithelial cells undergo a significant transcriptional reprogramming shortly after infection with S. Typhimurium that is strictly dependent on the function of its SPI-1 T3SS"
Based on this I would consider the promoter of a secreted effector?
Is this a specific gene?-there are a number of promoters in Salmonella-hilA is associated with a number of virulence genes, but there are a number of regulators and promoters