What is Positive control: CAP and cAMP? — Biology

Definition 20.5 University Biology — Year 1 · Chapter 20 — Control of Gene Expression

The lac promoter is weak: even without the repressor, the polymerase starts rarely unless a second protein, CAP (catabolite activator protein), is bound just upstream of it, bending the DNA and holding the polymerase in place. CAP binds DNA only when carrying cyclic AMP, whose level in the cell falls when glucose is abundant. The operon is therefore fully on only when lactose is present and glucose is absent: two signals, one negative and one positive, are integrated at one promoter. Given both sugars, E. coli eats the glucose first, pauses while cAMP rises and the lac enzymes are made, then eats the lactose — the two-phase growth curve called diauxie, which Monod described in 1941 and which set him on the road to the operon.

Diauxic growth of E. coli on glucose plus lactose. The culture grows on glucose alone, stops when it runs out, and resumes on lactose after the half-hour it takes to induce the lac operon. — Diauxic growth of *E. coli* on glucose plus lactose. The culture grows on glucose alone, stops when it runs out, and resumes on lactose after the half-hour it takes to induce the lac operon.

The lac operon made visible: colonies on a plate containing X-gal, a colourless substrate that -galactosidase turns blue. Blue colonies express lacZ; white ones carry a broken gene — the reporter used in a thousand experiments. — The lac operon made visible: colonies on a plate containing X-gal, a colourless substrate that $\beta$ -galactosidase turns blue. Blue colonies express *lacZ*; white ones carry a broken gene — the reporter used in a thousand experiments.

Examples

Example 20.7 (Two partial diploids)

$I^-\,O^+\,Z^+ / I^+\,O^+\,Z^-$ : the $I^+$ copy makes repressor that binds the $O^+$ next to $Z^+$ ; inducible — the mutation is complemented. $I^+\,O^c\,Z^+ / I^+\,O^+\,Z^-$ : the only working $Z$ sits behind an operator the repressor cannot bind; constitutive — the normal operator on the other copy cannot help, since it controls only the broken $Z$ .

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Examples

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