InterPro domain: IPR008434

This family represents the oxidative cyclase responsible for forming the distinctive E-ring of the chlorin ring system under aerobic conditions [ 1 ]. This enzyme is believed to utilise a binuclear iron centre and molecular oxygen. There are two isoforms of this enzyme in some plants and cyanobacteria, which are differentially regulated based on the levels of copper and oxygen [ 2 , 3 ]. This step is essential in the biosynthesis of both bacteriochlorophyll and chlorophyll under aerobic conditions (a separate enzyme, BchE, acts under anaerobic conditions). This enzyme is found in plants, cyanobacteria and other photosynthetic bacteria.

CRD1 (AcsF) is required for the maintenance of photosystem I and its associated light-harvesting complexes in copper-deficient (-Cu) and oxygen-deficient (-O(2)) Chlamydomonas reinhardtii cells and is localised to the thylakoid membrane. The family also contains the Rhodocyclus gelatinosus (Rhodopseudomonas gelatinosa or Rubrivivax gelatinosus) AcsF protein, which codes for a conserved, putative binuclear iron-cluster-containing protein involved in aerobic oxidative cyclization of Mg-protoporphyrin IX monomethyl ester. AcsF and homologs have a leucine zipper and two copies of the conserved glutamate and histidine residues predicted to act as ligands for iron in the Ex(29-35)DExRH motifs. Several homologs of AcsF are found in a wide range of photosynthetic organisms, including Chlamydomonas reinhardtii Crd1 and Pharbitis nil PNZIP, suggesting that this aerobic oxidative cyclization mechanism is conserved from bacteria to plants [ 4 ].

1. Rubrivivax gelatinosus acsF (previously orf358) codes for a conserved, putative binuclear-iron-cluster-containing protein involved in aerobic oxidative cyclization of Mg-protoporphyrin IX monomethylester. J. Bacteriol. 184, 746-53
2. The Crd1 gene encodes a putative di-iron enzyme required for photosystem I accumulation in copper deficiency and hypoxia in Chlamydomonas reinhardtii. EMBO J. 19, 2139-51
3. Reciprocal expression of two candidate di-iron enzymes affecting photosystem I and light-harvesting complex accumulation. Plant Cell 14, 673-88