Mg protoporphyrin monomethylester (MgProtoME) cyclase catalyzes isocyclic ring formation to form divinyl protochlorophyllide. The CHL27 protein is part of the cyclase complex. Deficiency of CHL27 has been previously reported to compromise photosynthesis and nuclear gene expression. In a comprehensive analysis of different CHL27 antisense tobacco lines grown under different light conditions, the physiological consequences of gradually reduced CHL27 expression on the tetrapyrrole biosynthetic pathway were explored. Excessive amounts of MgProtoME, the substrate of the cyclase reaction, accumulated in response to the reduced CHL27 content. Moreover, 5-aminolevulinic acid (ALA) synthesis, Mg chelatase and Mg protoporphyrin methyltransferase activities were reduced in transgenic plants. Compared with growth under continuous light exposure, the CHL27-deficient plants showed a stronger reduction in Chl content, cell death and leaf necrosis during diurnal light/dark cycles. This photooxidative phenotype correlated with a rapidly increasing MgProtoME steady-state level at the beginning of each light period. In contrast, the same transformants grown under continuous light exposure possessed a permanently elevated amount of MgProtoME. Its lower phototoxicity correlated with increased activities of ascorbate peroxidase and catalase, and a higher amount of reduced ascorbate. It is proposed that improved stress acclimation during continuous light in comparison with light-dark growth increases the capacity to prevent photooxidation by excess tetrapyrrole precursors and lowers the susceptibility to secondary photodynamic damage.