Operons

lac Operon Structural Genes Involved

lacZ, lacY, and lacA

trp Operon Structural Genes Involved

trpE, trpD, trpC, trpB, and trpA

ara Operon Structural Genes Involved

araB, araA, and araD

lac Operon Repressor

lacI - active when no lactose

lac Operon Inducer

Allolactase

trp Operon Repressor

trpR

trp Operon Corepressor

tryptophan

ara Operon Repressor

araC - in absence of arabinose

ara Operon Inducer

L-arabinose

lac Operon Mechanism of Control

Inducible Operon (usually OFF) lacI blocks transcription. When lactose is present, allolactose inactivates the repressor, turning the operon ON.

lac CAP/cAMP

CAP-cAMP acts as a positive regulator

low glucose → high cAMP → CAP binds promoter and boosts transcription

high glucose → low cAMP → weak transcription even if lactose is present

trp Mechanism of Control

Repressible operon, usually ON so cell can make tryptophan. When tryptophan is abundant, it binds trpR, activating the repressor and shutting it OFF.

Attenuation based on tryptophan levels.

ara Operon Mechanism of Control

Dual control system (inducible/repressible) Without arabinose, araC blocks transcription (DNA looping). With arabinose, araC changes shape and activates transcription.

CAP-cAMP enhances transcription when glucose is low (low glucose → high cAMP → CAP binds promoter → increased expression)