SUBSTRATE-DEPENDENT TRANSPORT OF THE NADPH-PROTOCHLOROPHYLLIDE OXIDOREDUCTASE INTO ISOLATED PLASTIDS

The key regulatory enzyme of chlorophyll biosynthesis in higher plants , the light-dependent NADPH:protochlorophyllide oxidoreductase (FOR), is a nuclear-encoded plastid protein. Its post-translational transport into plastids is determined by its substrate. The precursor of FOR (p POR) is taken up and processed to mature size by plastids only in the presence of protochlorophyllide (Pchlide). In etioplasts, the endogeno us level of Pchlide saturates the demands for pPOR translocation. Duri ng the light-induced transformation of etioplasts into chloroplasts, t he Pchlide concentration declined drastically, and isolated chloroplas ts rapidly lost the ability to import the precursor enzyme. The chloro plasts' import capacity for the pPOR, however, was restored when their intraplastidic level of Pchlide was raised by incubating the organell es in the dark with delta-aminolevulinic acid, a common precursor of t etrapyrroles. Additional evidence for the involvement of intraplastidi c Pchlide in regulating the transport of pPOR into plastids was provid ed by experiments in which barley seedlings were grown under light/dar k cycles. The intraplastidic Pchlide concentration in these plants und erwent a diurnal fluctuation, with a minimum at the end of the day and a maximum at the end of the night period. Chloroplasts isolated at th e end of the night translocated pPOR, whereas those isolated at the en d of the day did not. Our results imply that the Pchlide-dependent tra nsport of the pPOR into plastids might be part of a novel regulatory c ircuit by which greening plants fine tune both the enzyme and pigment levels, thereby avoiding the wasteful degradation of the imported pPOR as well as photodestruction of free Pchlide.