The TetR system we hoped to develop would have been similar to the lacI system. The pTet promoter which controls mtrB expression is regulated by the repressor TetR ([http://partsregistry.org/wiki/index.php?title=Part:BBa_C0040| BBa_C0040]). Expression of mtrB would have occurred when the system is induced with anhydrotetracycline which binds TetR. Unfortunately, as mtrB is toxic, we could not clone successful clone this system.