16.3.2—Repressible enzymes vs inducible enzymes

An inducible system is usually off until a substrate or signal removes repression and allows enzymes to be made. A repressible system is usually on until an end product activates repression and prevents unnecessary synthesis.

Both designs conserve energy by matching enzyme production to pathway need. The same word ‘repressor’ can participate in opposite logic depending on the signal and operator state.

A substrate may bind a repressor and stop it binding DNA in an inducible pathway; an accumulated product may bind a repressor and enable it to block a biosynthetic pathway.

Inducible does not mean permanently active, and repressible does not mean permanently off. State the starting state and the signal that changes it.