Inducers and corepressors Whether a regulatory protein is active sometimes depends on whether it is bound to a small molecule. Small molecules that bind to proteins and change their properties are called effectors An effector that binds to a repressor or activator and thereby initiates transcription of an operon is called the inducer of the operon. In contrast, an effector that binds to a repressor and causes it to block transcription is called a corepressor The activity of regulatory proteins is not necessarily altered only by binding to small molecule effectors. Some repressors and activators are covalently altered under some conditions for example, by methylation or phosphorylation• Inducers and Corepressors • Whether a regulatory protein is active sometimes depends on whether it is bound to a small molecule. Small molecules that bind to proteins and change their properties are called effectors. An effector that binds to a repressor or activator and thereby initiates transcription of an operon is called the inducer of the operon. In contrast, an effector that binds to a repressor and causes it to block transcription is called a corepressor. • The activity of regulatory proteins is not necessarily altered only by binding to small molecule effectors. Some repressors and activators are covalently altered under some conditions, for example, by methylation or phosphorylation