The C-terminal extension of ferrochelatase is critical for enzyme activity and for functioning of the tetrapyrrole pathway in Synechocystis strain PCC 6803

Heme and chlorophyll (Chl) share a common biosynthetic pathway up to the branch point where magnesium chelatase and ferrochelatase (FeCH) insert either magnesium for Chl biosynthesis or ferrous iron for heme biosynthesis. A distinctive feature of FeCHs in cyanobacteria is their C-terminal extension, which forms a putative transmembrane segment containing a Chl-binding motif. We analyzed the ΔH324 strain of Synechocystis sp. strain PCC 6803, which contains a truncated FeCH enzyme lacking this C-terminal domain. Truncated FeCH was localized to the membrane fraction, suggesting that the C-terminal domain is not necessary for membrane association of the enzyme. Measurements of enzyme activity and complementation experiments revealed that the ΔH324 mutation dramatically reduced activity of the FeCH, which resulted in highly upregulated 5-aminolevulinic acid synthesis in the ΔH324 mutant, implying a direct role for heme in the regulation of flux through the pathway. Moreover, the ΔH324 mutant accumulated a large amount of protoporphyrin IX, and levels of Chl precursors were also significantly increased, suggesting that some, but not all, of the “extra” flux can be diverted down the Chl branch. Analysis of the recombinant full-length and truncated FeCHs demonstrated that the C-terminal extension is critical for activity of the FeCH and that it is strictly required for oligomerization of this enzyme. The observed changes in tetrapyrrole trafficking and the role of the C terminus in the functioning of FeCH are discussed.