CAROTENOID-DEFICIENT YOUNG WHEAT ETIOPLASTS ARE ABLE TO BIND PRECURSOR PROTEINS ON THE PLASTID SURFACE BUT ARE IMPAIRED IN THEIR TRANSLOCATION ABILITY

Young carotenoid-deficient etioplasts, isolated from Norflurazon (NF)- treated wheat seedlings, were used to study the role of coloured carot enoids in the binding and import reactions of different nuclear-encode d plastid proteins. Plastids from control seedlings exhibited signific antly higher import efficiencies than did plastids from NF-treated pla nts. Etioplasts containing normal levels of carotenoids imported appro ximately 2000 and 800 molecules per plastid of the precursors of the s mall Rubisco subunit (pSS) and the Rieske FeS protein (pFeS), respecti vely. Plastids from NF-treated plants imported approximately 100 and 7 0 pSS and pFeS molecules per plastid, respectively. In addition, a max imum binding capacity of NF-treated plastids of 1200 protein molecules per plastid was observed for both pSS and pFeS when assayed at 25 deg rees C; and a maximum binding capacity of approximately 1300 molecules per plastid was noted at 4 degrees C. For control plastids, a similar amount of binding, or approximately 1400 protein molecules per plasti d, could only be observed if import was inhibited by low ATP concentra tions at 4 degrees C. When these plastids were washed and transferred to conditions promoting import at 25 degrees C and 10 mM Mg-ATP, close to 60% of the envelope-associated precursor protein molecules were im ported. These results indicate that control and NF-treated young etiop lasts contain similar amounts of binding sites far precursor proteins. However, only in the case of control plastids the binding was product ive and lead to import and processing in the stroma upon transfer to c onditions promoting import. Plastids isolated from wheat seedlings gro wn in weak red Light and containing different amounts of carotenoids, were assayed for their ability to bind and import a protein with unusu al import characteristics, the Chlamydomonas reinhardtii PsaF precurso r of PSI (pPsaF) and transit peptide deletion constructs. The PsaF pro tein was imported in a transit peptide-dependent manner into control e tioplasts, whereas import of pPsaF into young wheat etioplasts isolate d from NF-treated plants was inhibited at low levels of plastid carote noids.