A substance that promotes the activity of a gene or block of genes. Inducers of enzymes in prokaryotes fall into two groups. In negative control systems (e.g. the lac operon of Escherichia coli) they may be substrates of the enzymes they induce or analogues, derivatives, or precursors of the enzyme substrates. In any event, they bind with a regulator protein and so prevent the latter complexing with the operator gene to inhibit mRNA synthesis (see operon
). In positive control systems (e.g. the ara operon in E. coli) the inducer is a complex formed from the regulator-protein substrate. This complex must bind with the operator in order for mRNA synthesis to occur. In eukaryotes the nature of inducers is less well understood but appears more diverse. Steroid and other membrane-soluble growth substances, such as gibberellic acid, can act as inducers when complexed to cytoplasmic receptors.