Two different genes encode ferrochelatase in Arabidopsis: mapping, expression and subcellular targeting of the precursor proteins

Ferrochelatase is the last enzyme of haem biosynthesis. We have isolated 27 independent ferrochelatase cDNAs from Arabidopsis thaliana by functional complementation of a yeast mutant. Twenty-two of these cDNAs were similar to a previously isolated clone, AF3, and although they varied in length at the 5' and 3' ends, their nucleotide sequences were identical, indicating that they were derived from the same gene (ferrochelatase-I). The remaining five cDNAs all encoded a separate ferrochelatase isoform (ferrochelatase-II), which was 69% identical at the amino acid level to ferrochelatase-I. Using RFLP analysis in recombinant inbred lines, the ferrochelatase-I gene was mapped to chromosome V and that for ferrochelatase-II to chromosome II. Northern analysis showed that both ferrochelatase genes are expressed in leaves, stems and flowers, and expression in the leaves is higher in the light than in the dark. However, in roots only ferrochelatase-I transcripts were detected. High levels of sucrose stimulated expression of ferrochelatase-I, but had no effect, or repressed slightly, the expression of the ferrochelatase-II isoform. Import experiments into isolated chloroplasts and mitochondria showed that the ferrochelatase-II gene encodes a precursor which is imported solely into the chloroplast, in contrast to ferrochelatase-I which is targeted to both organelles. The significance of these results for haem biosynthesis and the production of haemoproteins, both within the plant cell and in different plant tissues, is discussed.