In Arabidopsis thaliana, we have found a negative regulator of tetrapyrrole biosynthesis, FLU, which operates independently of heme and seems to selectively affect only the Mg2 branch of tetrapyrrole biosynthesis. The identity of this protein was established by map-based cloning and sequencing the FLU gene. FLU is a nuclear-encoded plastid protein that, after import and processing, becomes tightly associated with plastid membranes. It is unrelated to any of the enzymes known to be involved in tetrapyrrole biosynthesis.

1. Ethiolated flu mutant glows due excess of the chlorophyll precursor, protochlorophyllide flu mutant Wild type

2. Excess of protochlorophyllide kills the dark – grown mutant in the light flu mutant Wild type

3. In continuos light the mutant converts protochlorophyllide into chrolophyll and survives flu mutant Wild type

4. Flu domain architecture and mutations C -terminus N -terminus flu 1-3 R  stop CTP HR CC-D TPR-D Chloroplast transit peptide

5. Flu domain architecture and mutations Coiled – coil domain C -terminus flu 1-4 A  V N -terminus CTP HR CC-D TPR-D Hydrophobic region

6. Flu domain architecture and mutations C -terminus flu 1-1 A  V N -terminus flu 1-2 Q  stop CTP HR CC-D TPR-D Tetratricopeptide repeat domain