Phytochrome-mediated repression of enzyme synthesis (lipoxygenase): a threshold phenomenon.

Synthesis of the enzyme lipoxygenase in the cotyledons of the mustard seedling (Sinapis alba L.) is controlled by phytochrome (P(fr)) through a threshold mechanism. The repression of enzyme synthesis by P(fr) is a very rapid process after the threshold level is surpassed. Similarly, enzyme synthesis starts instantaneously and with full speed as soon as the P(fr) level decreases below the threshold level. Thus P(fr) rapidly inhibits synthesis of an enzyme and functions through an all-or-none control mechanism of high precision.