For example, if a given metabolite is not present, enzymes for its breakdown are not necessary, and synthesizing these enzymes is wasteful If a cell produces enzymes for the degradation of a particular carbon source only when this carbon source is present in the environment, the enzyme system is known as an inducible system.

In contrast, the enzymes in many synthetic pathways are in low concentration or absent when an adequate quantity of the end product of the pathway is already available to the cell. That is, if the cell encounters an abundance of the amino acids, e.g. tryptophan in the environment or if it is over produced, the cell stops the manufacture of tryptophan until a need arises again.

A repressible system is a system of enzymes whose presence is repressed, stopping the production of the end product when it is no longer needed. Repressible systems are repressed by an excess of the end product of their synthetic pathway.Bacteria regulate or alter the gene expression either by using positive or negative regulation.