Some bacteria can synthesize the amino acid isoleucine from threonine, a process involving five enzymes ($E_1$ to $E_5$) and four intermediary products (P, Q, R and S). The production of isoleucine is controlled by end-product inhibition.

Some bacteria can synthesize the amino acid isoleucine from threonine, a process involving five enzymes ($E_1$ to $E_5$) and four intermediary products (P, Q, R and S). The production of isoleucine is controlled by end-product inhibition.

Which statement describes this end-product inhibition?