2 NADPH-PROTOCHLOROPHYLLIDE OXIDOREDUCTASES IN BARLEY - EVIDENCE FOR THE SELECTIVE DISAPPEARANCE OF PORA DURING THE LIGHT-INDUCED GREENING OF ETIOLATED SEEDLINGS

Chlorophyll synthesis in barley is controlled by two different light-d ependent NADPH:protochlorophylide oxidoreductases, termed PORA and POR E, PORA is present abundantly in etioplasts but selectively disappears soon after the beginning of illumination, This negative light effect is mediated simultaneously at three different levels, First, the conce ntration of porA mRNA declines drastically during illumination of dark -grown seedlings, Second, the plastids' ability to import the precurso r of PORA (pPORA) is reduced during the transition from etioplasts to chloroplasts. This effect is due to a rapid decline in the plastidic l evel of protochlorophyllide (Pchlide), which is required for the trans location of the pPORA, Third, PORA becomes selectively destabilized in illuminated seedlings, When illuminated, PORA-Pchlide-NADPH complexes formed in the dark photoreduce their Pchlide to Chlide and become sim ultaneously susceptible to attack by plastid proteases, The PORA-degra ding protease activity is not detectable in etioplasts but is induced during illumination, In contrast to PORA, the second Pchlide-reducing enzyme, PORE, remains operative in both illuminated and green plants, Its translocation into plastids does not depend on its substrate, Pchl ide.