A process in which the de novo synthesis of a particular enzyme or group of enzymes is stimulated by the presence of the substrate, especially when the substrate is the cell's only carbon source. The induction of enzymes has been studied for the most part in bacteria. The classical example of enzyme induction is that of β-galactosidase in Escherichia coli. This enzyme rapidly increases in activity when E. coli cells are given lactose as the sole carbon source. This induction effect is not seen, however, if glucose is present, even if lactose is present in high concentrations. In eukaryotic cells induction is not as common as in bacteria, nor is it so rapid. In primitive eukaryotes, such as Saccharomyces and Neurospora, induction of β-galactosidase can be observed, although at a much slower rate than in bacteria. In higher eukaryotes there are fewer proven examples of enzyme induction. One that has been studied in detail in white mustard (Sinapis alba) involves the synthesis of phenylalanine ammonialyase (PAL), which catalyses the formation of cinnamic acid (a precursor of the flavonoids) from phenylalanine. Transcription of the gene governing PAL synthesis is induced by the Pfr form of phytochrome. See also operon. |
