Mutation in a chlorophyll-binding motif of Brassica ferrochelatase enhances both heme and chlorophyll biosynthesis

The heme branch of tetrapyrrole biosynthesis contributes to the regulation of chlorophyll levels. However, the mechanism underlying the balance between chlorophyll and heme synthesis remains elusive. Here, we identify a dark green leaf mutant, dg, from an ethyl methanesulfonate (EMS)-induced mutant library of Chinese cabbage. The dg phenotype is caused by an amino acid substitution in the conserved chlorophyll a/b-binding motif (CAB) of ferrochelatase 2 (BrFC2). This mutation increases the formation of BrFC2 homodimer to promote heme production. Moreover, wild-type BrFC2 and dBrFC2 interact with protochlorophyllide (Pchlide) oxidoreductase B1 and B2 (BrPORB1 and BrPORB2), and dBrFC2 exhibits higher binding ability to substrate Pchlide, thereby promoting BrPORBs-catalyzed production of chlorophyllide (Chlide), which can be directly converted into chlorophyll. Our results show that dBrFC2 is a gain-of-function mutation contributing to balancing heme and chlorophyll synthesis via a regulatory mechanism in which dBrFC2 promotes BrPORB enzymatic reaction to enhance chlorophyll synthesis. [Display omitted] •A nonsynonymous homozygous mutation of BrFC2 results in increased heme and chlorophyll•BrFC2 and mutated dBrFC2 interact with BrPORBs•The mutated dBrFC2 facilitates Pchlide-binding ability to promote chlorophyll synthesis•BrFC2 and BrPORBs coordinately influence chlorophyll and heme biosynthesis Liu et al. identify a nonsynonymous homozygous mutation of BrFC2 resulting in increased heme and chlorophyll. BrFC2 and mutated dBrFC2 interact with BrPORBs, and the mutated dBrFC2 facilitates Pchlide-binding ability to promote chlorophyll synthesis. dBrFC2 is a “gain-of-function mutation” contributing to the balance between heme and chlorophyll synthesis.