IMPORT OF NUCLEAR-ENCODED PROTEINS INTO CAROTENOID-DEFICIENT YOUNG ETIOPLASTS

Young etioplasts with different carotenoid contents were assayed for t heir ability to import in vitro synthesized nuclear-encoded proteins. The plastids were isolated from the basal 1.5 cm of dark-grown wheat s eedlings developed from seeds imbibed with 4 different concentrations of Norflurazon, an inhibitor of the carotenoid biosynthesis. Plastids isolated from plants treated with the two highest concentrations, 2.8 and 28 mg l-1, of Norflurazon contained approximately 10 and 5% of the carotenoid contents, respectively, compared to the control. The total amounts of proteins in these plastids were approximately 68 and 60% c ompared to control plastids. Translocation assays employing the precur sors of the small subunit of ribulose-1,5-bisphosphate carboxylase/oxy genase (pSS), and the non-photosynthetic heat-shock protein 21 (pHSP21 ), showed that the rate of protein import was considerably lower in pl astids with low carotenoid contents. The amounts of imported, processe d SS were 11 and 10% after 2.8 and 28 mg l-1, respectively, compared t o the control, whereas the amounts of HSP21 at these herbicide concent rations were 20 and 18%, respectively. The low apparent import in plas tids of Norflurazon-treated leaves was not an effect of intraorganella r degradation of imported proteins, nor were there any differences in the amounts of processed, protease-protected protein when Norflurazon was added to the import reaction using control plastids. The low impor t capabilities are therefore discussed in relation to the possible rol e of the carotenoids in the translocation of cytosolically synthesized proteins into the plastidic compartment.