A PLASTID ENZYME ARRESTED IN THE STEP OF PRECURSOR TRANSLOCATION IN-VIVO

The key enzyme of chlorophyll biosynthesis in higher plants, NADPH:pro tochlorophyllide (Pchlide) oxidoreductase (FOR, EC 1.3.1.33), accumula tes in its precursor form (pPORA) in barley. pPORA is bound to the chl oroplasts and is able to interact with the enzyme's substrate, Pchlide , at both the cytosolic as well as the stromal side of the plastid env elope. The interaction with intraplastidic Pchlide, formed in ATF-cont aining chloroplasts upon feeding with delta-aminole-vulinic acid, driv es vectorial translocation of pPORA across the plastid envelope membra nes. In contrast, exogenously applied Pchlide causes the release of th e envelope-hound precursor protein to the cytosol. Both processes comp ete with each other if intra- and extraplastidic Pchlide are applied s imultaneously. A cytosolic heat shock cognate protein of M(r) 70,000 p resent in wheat germ and barley leaf protein extracts appears to preve nt the release of the pPORA to the cytosol in vivo, however.