This protein, coded by a single gene in Arabidopsis, is associated with 2 chloroplast membrane systems: the membranes enclosing the chloroplast - envelope - and the photosynthetic membranes - thylakoids -. Deletion of the gene encoding this protein blocks the development of photosystems; absence of the protein causes an accumulation of Mg protoporphyrin IX and an increased inhibition of nuclear gene expression for photosystem proteins. This indicates that not only Mg protoporphyrin IX inhibits the expression of photosynthesis genes, as shown in a previous study, but that the methylated product of the Mg protoporphyrin IX or one of its derivatives plays a role in upregulation of photosynthetic gene expression. In addition, the absence of methyltransferase stabilises a high turn-over sub-unit of another enzyme of the chlorophyll synthesis pathway (CHLH) and this subunit accumulates within the chloroplast. Further research is being carried out to establish whether this stabilisation is linked to the accumulation of Mg protoporphyrin IX, as would seem likely.