Substrate channeling between the enzymatic steps in the (bacterio)chloro-phyll biosynthetic pathway catalyzed by magnesium chelatase (BchI⁄ChlI, BchD⁄ChlD and BchH⁄ChlH subunits) andS-adenosyl-l-methionine:mag-nesium-protoporphyrin IX O-methyltransferase (BchM⁄ChlM) has been suggested. This involves delivery of magnesium-protoporphyrin IX from the BchH⁄ChlH subunit of magnesium chelatase to BchM⁄ChlM.