Phenylalanine ammonia-lyase and cinnamic acid 4-hydroxylase: Characterisation of the concomitant changes in enzyme activities in illuminated potato tuber discs.

Phenylalanine ammonia-lyase (PAL, EC 4.3.1.5) and cinnamic acid 4-hydroxylase (CA4H, EC 1.14.13.11) undergo concomitant increases in activity after a 2 h lag following disc preparation and illumination. The increases in PAL and CA4H activity can be inhibited by actinomycin-D, cordycepin and cycloheximide. It is demonstrated that the concomitant increases in enzyme activity stem from the simultaneous stimulation of the respective actinomycin-D-sensitive steps followed by parallel transmission of these stimuli through the cordycepin- and cycloheximide-sensitive steps. The events between the actinomycin-D- and cordycepin-sensitive steps are rate limiting in the expression of the stimulations in terms of increased enzyme activities. Delayed transfer of discs to actinomycin-D or cordycepin 'superinduces' PAL but not CA4H activity. This superinduction is related to the inhibition by cycloheximide of the subsequent decline in PAL activity and is interpreted in terms of the transcription and translation of the mRNA coding for an inactivator-protein of PAL.