Gene Regulation in E.coli

	Positive Regulation	Negative Regulation
Function	Decisions about utilizing the best sources of C, N, electron donor, electron acceptor, etc.	Decisions about whether to synthesize something that might currently be available from the environment.
Regulatory protein	activator	repressor
Effectors: small signaling molecules	Inducer binds and activates the activator protein.	a. Co-repressors - some repressors may not function unless they are first bound to a small co-repressor molecule. b. Inducer - repression may be relieved when a small inducer binds and inactivates the repressor.
Relevant DNA sequences	Specific activator binding sites are recognized by activator proteins.	Specific operator sites are recognized by repressors.
Examples	a. The activator protein CAP (Catabolite Activator Protein) of E. coli interacts with the inducer cAMP. The protein subsequently binds CAP binding sites in DNA and stimulates transcription of downstream genes. This system is involved in the use of alternate carbon sources when the primary source, glucose, is in short supply. b. The activator protein virG of Agrobacterium tumefaciens is activated by phosphorylation in response to substances released from wounded plants, rather than being activated by the binding of an inducer. It subsequently directs the tryptophan and represses the transcription of genes encoding enzymes involved in tryptophan biosynthesis if glucose is not available.

modified from here.