Abstract
The unique isocyclic E ring of chlorophylls contributes to their role as light-absorbing pigments in photosynthesis. The formation of the E ring is catalyzed by the Mg-protoporphyrin IX monomethyl ester cyclase, and the O2-dependent cyclase in prokaryotes consists of a diiron protein AcsF, augmented in cyanobacteria by an auxiliary subunit Ycf54. Here, we establish the composition of plant and algal cyclases, by demonstrating the in vivo heterologous activity of O2-dependent cyclases from the green alga Chlamydomonas reinhardtii and the model plant Arabidopsis thaliana in the anoxygenic photosynthetic bacterium Rubrivivax gelatinosus and in the non-photosynthetic bacterium Escherichia coli. In each case, an AcsF homolog is the core catalytic subunit, but there is an absolute requirement for an algal/plant counterpart of Ycf54, so the necessity for an auxiliary subunit is ubiquitous among oxygenic phototrophs. A C-terminal ∼40 aa extension, which is present specifically in green algal and plant Ycf54 proteins, may play an important role in the normal function of the protein as a cyclase subunit.
Highlights
All chlorophototrophic organisms rely on the unique chemical properties ofchlorophyll [(B)Chl] molecules for light harvesting and photochemical reactions, so the elucidation of the (B)Chl biosynthesis pathways is of great importance
In the common (B)Chl biosynthetic pathway, the least wellcharacterized step is the formation of the isocyclic E ring, via the oxidation and cyclization of the C13 methyl propionate group of Mg-protoporphyrin IX monomethyl ester (MgPME), producing 3,8-divinyl protochlorophyllide a (DV PChlide a)
We did not detect any cyclase activity of Chlamydomonas CTH1 co-expressed with CGL78 in E. coli (Figure 5), possibly because CTH1 was not expressed or it was expressed but unstable in E. coli
Summary
All chlorophototrophic organisms rely on the unique chemical properties of (bacterio)chlorophyll [(B)Chl] molecules for light harvesting and photochemical reactions, so the elucidation of the (B)Chl biosynthesis pathways is of great importance. In the common (B)Chl biosynthetic pathway, the least wellcharacterized step is the formation of the isocyclic E ring, via the oxidation and cyclization of the C13 methyl propionate group of Mg-protoporphyrin IX monomethyl ester (MgPME), producing 3,8-divinyl protochlorophyllide a (DV PChlide a). This step is catalyzed by two mechanistically unrelated cyclases: an O2-sensitive, radical SAM enzyme containing [4Fe–4S] and cobalamin cofactors [1] is encoded by the bchE gene in most anoxygenic phototrophic bacteria and some cyanobacteria [2], whereas an O2-dependent diiron monooxygenase is found in many purple bacteria, as well as cyanobacteria, algae and plants [3]. PCC 6803 (hereafter Synechocystis) [7], completing the identification of the O2-dependent cyclase components in prokaryotic phototrophs
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