A SINGLE PRECURSOR PROTEIN FOR FERROCHELATASE-I FROM ARABIDOPSIS IS IMPORTED IN-VITRO INTO BOTH CHLOROPLASTS AND MITOCHONDRIA

Ferrochelatase is the last enzyme of heme biosynthesis and in higher p lants is found in both chloroplasts and mitochondria, We have isolated cDNAs for two isoforms of ferrochelatase from Arabidopsis thaliana, b oth of which are imported into isolated chloroplasts. In this paper we show that ferrochelatase-I is also imported into isolated pea mitocho ndria with approximately the same efficiency as into chloroplasts. Pro cessing of the precursor was observed with both chloroplast stroma and mitochondrial matrix extracts. This was inhibited by EDTA, indicating it was due to the specific processing proteases, The specificity of i mport was verified by the fact that the mitochondrial preparation did not import the precursor of the light-harvesting chlorophyll alb prote in precursor or the precursor of porphobilinogen deaminase, an earlier enzyme of tetrapyrrole biosynthesis, both of which are exclusively ch loroplast-located, Furthermore, import of ferrochelatase-I precursor i nto mitochondria was inhibited by valinomycin, but this had no effect on its import into chloroplasts. Thus a single precursor molecule is r ecognized by the import machinery of the two organelles, The implicati ons for the targeting of ferrochelatase in a possible protective role against photooxidative stress are discussed.