Interaction between chloroplast and cytoplasmic factors in the inhibition of l ‐phenylalanine ammonia‐lyase activity

Components of the system inactivating cytoplasmic l‐phenylalanine ammonia‐lyase (PAL; EC 4.3.1.5.) in barley seedlings (Hordeiim vulgare L. cv. Polon) were investigated. In vitro light‐activated chloroplasts and two cytoplasmic factors participate in the enzyme inhibition. One role of the chloroplasts is to mediate the reaction between PAL and the cytoplasmic factors, resulting in inactivation of the enzyme. Uncoupling (carbonyl cyanide m‐chlorophenyl hydrazone, valinomycin) or inhibiting [3‐(3,4‐dichlorophenyl)‐l,l‐dimethylurea] photosynthetic electron transport showed that electron transport is necessary to enable chloroplasts to mediate PAL inhibition. Preliminary results also indicate that mitochondria are able to mediate the inhibition. The role of this inhibiting system in the regulation of PAL activity in vivo is discussed.