Heme Synthesis
Tingting Fan, Lena Roling, Daniel Hey, Patricia Ortega-Rodés, Boris Hedtke
In plants, the entire chlorophyll and heme-synthesizing pathway occurs in plastids. It was proposed that a second site for heme biosynthesis in plants exclusively consists of the last two enzymes, protoporphyrinogen oxidase and ferrochelatase, in mitochondria. Two genes encode the distinct isoforms of these two enzymes. While protoporphyrinogen oxidase I and ferrochelatase II are exclusively targeted to plastids, dual-targeting of the each second isoform to plastids and mitochondria is still not excluded.
Ferrochelatase is a last enzyme of protoheme biosynthesis in plants. In Arabidopsis the two genes encoding ferrochelatase 1 and 2 (FC1 and FC2) show different expression patterns. FC2 predominantly accumulates in leaves, stems and flowers, while FC1 is expressed mainly in roots, but to a much lower level than FC2 in leaves.
Although both ferrochelatases share high structural and catalytic similarities, they are reported to serve different heme pools. A knockout of the FC1 gene causes embryo lethality in Arabidopsis, while fc2 knockout mutants cannot survive under short day condition and form small, pale green leaves under long day and continuous light condition. These observations indicate that the two Arabidopsis ferrochelatase isoforms (FC1 or FC2) cannot apparently substitute each other’s function.
One of the aims of our research is to investigate the functional similarities and differences between FC1 and FC2. One of the approaches is to examine complementation strategies of both FCs knockout mutants by expressing the two FC variants as well as hybrids of both coding sequences under different endogenous and constitutive promoters. A second objective is to clarify the subcellular localization of each ferrochelatase isoform and verify the contribution of each isoform to the heme supply for chloroplasts, mitochondria and cytoplasm.
Scheme of the Arabidopsis gene encoding ferrochelatase 1 (AtFeCh1) and the T-DNA insertion site in the third exon of the fc1 null mutant.
A silique of the Arabidopsis fc1 mutant with 25% embryo lethality.
Two different fc2 mutants in comparison to wild-type seedlings grown under continuous light.