Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transfer
Protein Synthesis and Its Regulation
Enzyme repression also occurs in prokaryotes

Another important type of Enzyme Concentration change in a bacterial Cell, which is the opposite of enzyme induction, is enzyme repression. When E. coli Cells grow on a medium containing an ammonium salt as the sole nitrogen source, they must synthesize all nitrogen-containing compounds from the NH⁺₄ ion and a carbon source. Such cells obviously need to contain all the enzyme systems required to synthesize 20 different Amino Acids. However, if one of the amino acids, say Histidine, is added to the medium, The Cell stops producing the entire set of Enzymes necessary to synthesize histidine from ammonia and the carbon source, while continuing to synthesize enzyme sets for the remaining 19 amino acids. As the cell population continues to grow and divide, the specific activity of the remaining histidine-synthesizing enzymes decreases. The shutdown of the synthesis of enzymes responsible for histidine production, triggered by The addition of histidine, is called enzyme repression. Repression, much like induction, reflects THE PRINCIPLE OF cellular economy: as soon as the histidine Biosynthesis enzymes are no longer needed, their production is halted. In most cases, enzyme repression affects enzymes involved in biosynthesis, particularly Amino acid biosynthesis. The repression of an entire set of enzymes catalyzing a sequence of biosynthetic reactions by the end product of that pathway, as seen in the case of histidine-synthesizing enzymes, is termed coordinated repression or end-product repression.