Tet Repressor proteins (otherwise known as TetR) are proteins playing an important role in conferring antibiotic resistance to large categories of bacterial species. Tetracycline (Tc) is a broad family of antibiotics to which bacteria have evolved resistance. Tc normally kills bacteria by binding to the bacterial ribosome and halting protein synthesis. The expression of Tc resistance genes is regulated by the repressor TetR. TetR represses the expression of TetA, a membrane protein that pumps out substances toxic to the bacteria like Tc, by binding the tetA operator. In Tc-resistant bacteria, TetA will pump out Tc before it can bind to the ribosome because the repressive action of TetR on TetA is halted by binding of Tc to TetR. Therefore, TetR may have an important role in helping scientists to better understand mechanisms of antibiotic resistance and how to treat antibiotic resistant bacteria. TetR is one of many proteins in the TetR protein family, which is so named because TetR is the most well characterized member. TetR is used in artificially engineered gene regulatory networks because of its capacity for fine regulation of promoters. In the absence of Tc or analogs like ATc, basal expression of TetR-regulated promoters is low, but expression rises sharply in the presence of even a minute quantity of Tc. The tetA gene is also present in the widely used E. coli cloning vector pBR322, where it is often referred to by the name of its tetracycline-resistance phenotype, TetR, not to be confused with TetR. Structure & Function TetR functions as a homodimer. Each monomer consists of ten alpha helices connected by loops and turns. The overall structure of TetR can be broken down into two DNA-binding domains (one per monomer) and a regulatory core, which is responsible for tetracycline recognition and dimerization. TetR dimerizes by making hydrophobic contacts within the regulatory core. There is a binding cavity for tetracycline in the outer helices of the regulatory domain. When tetracycline binds this cavity, it causes a conformational change that affects the DNA-binding domain so that TetR is no longer able to bind DNA. As a result, TetA and TetR are expressed. There is still some debate in the field whether tetracycline derivatives alone can cause this conformational change or whether tetracycline must be in complex with magnesium to bind TetR. (TetR typically binds tetracycline-Mg2+ complexes inside bacteria, but TetR binding to tetracycline alone has been observed in