E. coli has a protein to protect other periplasmic proteins from low pH environments called the Asr protein. The gene responsible for this protein is PhoB-dependent, and can only be turned on when the Pho regulon is activated by low Pi concentration. Synthesis of the Asr protein imparts acid shock resistance to E. coli enabling it to survive in environments like the stomach which has a low pH. Many acid tolerance genes are induced by more than just the low pH environment and require other environmental signals to be present in order to be activated. These specific nutrients being present or in low concentrations, anaerobiosis, and host-produced factors.