Transcription regulator that can specifically activate or repress expression of target genes. Required for virulence and survival in the macrophage environment. Probably activates expression of ispA, xseB genes, and of omp operon. Homodimer. During infection of the host cells. Down-regulated by itself. Activated by Mg(2+) starvation in a phoP dependent manner. Belongs to the SlyA family. Was originally thought to be a toxin with hemolytic and cytolytic activity. Extended N-terminus. Extended N-terminus.