Three classes of oxygen-dependent cyclase involved in chlorophyll and bacteriochlorophyll biosynthesis

Significance (Bacterio)chlorophylls harvest and convert the solar energy that powers the biosphere. The absorption properties of these pigments are determined in part by formation of the isocyclic ring, which confers their characteristic green color. This last remaining uncharacterized biosynthetic step has remained enigmatic for more than 60 y, and only a single subunit of the O 2 -dependent cyclase, AcsF, has been identified. Here we demonstrate that Ycf54 is a bona fide subunit of this enzyme in oxygenic phototrophs; identify a new cyclase subunit, BciE, in Alphaproteobacteria; and confirm that the AcsF found in other classes of bacterial phototrophs is the principal form of the cyclase, requiring neither Ycf54 nor BciE for activity.