Aporepressor

In molecular biology, an aporepressor is a regulator protein that acts on an operon. Its activity is modulated by small molecules, such as metabolites, called corepressors. It is activated when its corepressor binds, at which point it binds at the operator sequence of the operon. By binding to the DNA, the operon is inhibited or completely inactivated.

In general, repressor is the preferred term for any protein that negatively regulates a gene or set of genes.

In the trp operon, the "trp" repressor is an aporepressor. In the presence of tryptophan, the "trp" repressor can bind the operator sequence and inhibit the transcription of the operon genes. Without tryptophan, the aporepressor undergoes a conformational change that hides its DNA binding sites. This enables the transcription of the tryptophan biosynthesis genes encoded in the "trp" operon.