Density-labelling evidence for the phytochrome-mediated activation of phenylalanine ammonia-lyase in mustard cotyledons

Abstract

When dark-grown mustard seedlings are irradiated with far-red light the level of phenylalanine ammonia-lyase (EC 4.3.1.5) activity increases. After 2H 2 O treatment phynlalanine amonia-lyase from seedlings irradiated with far-red light is density-labelled to a lesser extent than enzyme from dark-grown tissue. Theoretical arguments are advanced and data presented which show that this result cannot be explained in terms of an increase in de novo synthesis of phenylalanine ammonia-lyase and that the increase most likely involves activation of existing enzyme.