Global and local regulation of a metabolic pathway. Yellow (resp. pink) boxes represent metabolite (resp. enzyme) pools. The transcription factors are represented by ellipses. Green and red indicate that the transcription factor is "on" and "off", respectively, where "on" means that the transcription factor is able to bind to DNA. The local transcription factor (TF2) is sensitive to an intermediate metabolite (Xn), and modulates the synthesis of enzyme(s) involved in the pathway. The global regulator TF1, sensitive to another signal, can modulate (i) the synthesis of intermediate enzymes; (ii) the synthesis of the local transcription factor TF2, or (iii) both.