Regulation of enzyme levels in phenylpropanoid biosynthesis: Characterization of the modulation by light and pathway intermediates

Abstract

The accumulation of chlorogenic acid in disks of potato ( Solanum tuberosum cv King Edward) tuber tissue is controlled by rapid, interrelated changes in the activity levels of the biosynthetic enzymes phenylalanine ammonia-lyase (EC 4.3.1.5), cinnamic acid 4-hydroxylase (EC 1.14.13.11), and hydroxycinnamoyl-CoA:quinic acid hydroxycinnamoyl transferase (EC 2.3.1.-). The modulation of enzyme levels by light and by the pathway intermediates cinnamic acid and p-coumaric acid has been characterized by (a) effectiveness at various stages during the development of enzyme activity, (b) the kinetics of the regulatory responses, and (c) application of the inhibitors cordycepin, cycloheximide, and actinomycin-D. Using these criteria, clear mechanistic differences between modulation of enzyme levels by light and modulation by pathway intermediates have been observed. Photomodulation of enzyme levels occurs by a relatively slow, actinomycin-D-sensitive mechanism. In contrast, modulation by pathway intermediates occurs by a rapid, post-transcriptional mechanism acting at the rate-limiting step in the development of enzyme activity. This mechanism is consistent with the hypothesis that the coordination of enzyme levels is maintained by modulation by pathway intermediates.