MOLECULAR LOCALIZATION OF FERROCHELATASE IN HIGHER-PLANT CHLOROPLASTS

Within the chloroplast of higher plants, a crucial branchpoint of the tetrapyrrole synthesis pathway is the chelation of either Fe2+ to make haem, or Mg2+ for chlorophyll, catalysed by ferrochelatase or magnesi um chelatase, respectively. One model that has been proposed for the c ontrol of this branchpoint, based on biochemical studies, is that the two enzymes are spatially separated within the chloroplast, ferrochela tase being exclusively in the thylakoids, while magnesium chelatase is associated with the envelope [Matringe, M., Camadro, J.-M., Joyard, J . & Douce, R. (1994) J. Biol. Chem. 269, 15010-15015]. We have used a sensitive molecular method to investigate this possibility. Radiolabel led precursor proteins for ferrochelatase from Arabidopsis have been i mported into isolated chloroplasts. Their distribution in the differen t subchloroplastic fractions have then been determined, and compared w ith that for light-harvesting chlorophyll protein, which is exclusivel y thylakoidal, and the envelope-located phosphate translocator. Clear evidence for the specific association of ferrochelatase protein with b oth thylakoid and envelope membranes has been obtained, thus suggestin g strongly that the control of the branchpoint cannot, be by spatial s eparation of the two chelatases.