Some substances called ‘inducers’ are capable of inducing or activating the expression of genes. In the presence of such inducers, the genes are transcribed and translated to synthesize the enzymes, which then metabolize the inducer. This phenomenon is called ‘induction’ and the enzymes synthesized are called ‘inducible enzymes’. For example, when E. coli is grown in the presence of lactose as a carbon source, the lactose acts as inducer and activates the genes to synthesize the enzyme β-galactosidase (β-gal), which metabolizes the lactose to glucose and galactose.
On the other hand, certain substances are capable of reducing the expression of structural genes; such substances are called ‘repressors’ and the process of reducing the gene expression mediated by a repressor is called ‘enzyme repression’. Those metabolites whose introduction into a growth medium specifically decreases the amount of specific enzymes are called ‘co-repressors’. For example, the presence of large amounts of amino acids in the medium leads to the repression of the genes for the enzymes catalysing the biosynthesis of that amino acid in the bacteria.