Fig. 1: Regulatory networks for lactose and melibiose utilization in Escherichia coli.
From: Effects of resource packaging on the adaptative and pleiotropic consequences of evolution
a The lac operon includes lacZ (β-galactosidase) and lacY (lactose permease), whose expression is repressed by the LacI repressor protein in the absence of lactose. When lactose (white square linked with white circle) is present, it binds LacI, relieving repression and enabling transcription from the lacZY promoter. lacY-encoded permease facilitates lactose import, creating a positive feedback loop. Hydrolysis of lactose by LacZ releases glucose (white square) and galactose (white circle). LacI is also an autorepressor. b The mel operon includes melA (α-galactosidase) and melB (melibiose transporter), regulated by the activator MelR. In the presence of melibiose (rotated white square linked with white circle), MelR activates transcription of melA and melB, with MelB-mediated import enhancing inducer availability, forming a similar positive feedback loop as in the case of lactose. MelR is an auto-repressor. Hydrolysis of melibiose by MelB releases glucose (white square) and galactose (white circle).
