Catalyzes the formation of the isocyclic ring in chlorophyll biosynthesis. Mediates the cyclase reaction, which results in the formation of divinylprotochlorophyllide (Pchlide) characteristic of all chlorophylls from magnesium-protoporphyrin IX 13-monomethyl ester (MgPMME). 2 H(+) + Mg-protoporphyrin IX 13-monomethyl ester + 3 NADPH + 3 O2 = 3,8-divinyl protochlorophyllide a + 5 H2O + 3 NADP(+) Porphyrin-containing compound metabolism; chlorophyll biosynthesis. Part of the FLU-containing chloroplast membrane complex composed of FLU, CRD1, PORB, PORC, CHLP and HEMA1. A number of isoforms are produced. According to EST sequences. Knock-down mutant (chl27-t) grow slowly with a pale green appearance. confers also severe defects in chloroplast development, including the unstacking of thylakoid membranes (PubMed:18682427). Belongs to the AcsF family. Truncated N-terminus.