NADPH : protochlorophyllide oxidoreductase uses the general import route into chloroplasts

Chloroplast differentiation in angiosperm plants depends on the light-depen dent conversion of protochlorophyllide to chlorophyllide by NADPH:protochlo rophyllide oxidoreductase (PORA; EC 1.6.99.1), a nuclearly encoded protein. The protein import of the precursor form of PORA into plastids was shown p reviously to strictly depend on the presence of its substrate protochloroph yllide. PORA seemed to follow a novel, posttranslationally regulated import route. Here we demonstrate that the precursor of PORA from barley is impor ted into isolated barley plastids independently of protochlorophyllide. POR A as well as PORE import is competed for by the precursor of the small subu nit of Rubisco. The data demonstrate that the PORA precursor uses the gener al import pathway into plastids. Furthermore, en route into chloroplasts th e pea FOR precursor can be crosslinked to the protein import channel in the outer envelope Toc75 from pea.