Apart from the PhoP/PhoQ-regulated ugd ( pagA ) gene , which in serovar Typhimurium has been demonstrated to be transcriptionally active inside macrophages and to be necessary for growth in a low-magnesium environment ( 1 , 19 , 20 ) , two genes were identified with homology to the E. coli tdh gene and the cat2 gene in Clostridium aminobutyricum .