Role of magnesium chelatase activity in the early steps of the tetrapyrrole biosynthetic pathway

Magnesium-protoporphyrin IX chelatase (Mg-chelatase) is located at the bran chpoint of tetrapyrrole biosynthesis, at which point protoporphyrin IX is d istributed for the synthesis of chlorophyll and heme. We investigated the r egulatory contribution of Mg-chelatase to the flow of metabolites. In plant s, the enzyme complex consists of three subunits, designated CHL D, CHL I, and CHL H. Transgenic tobacco (Nicotiana tabacum) plants expressing antisen se RNA for the Mg-chelatase subunit CHL H were analyzed to elucidate furthe r the role of Mg-chelatase in the distribution of protoporphyrin IX into th e branched tetrapyrrolic pathway. The transgenic plants displayed a reduced growth rate and chlorophyll deficiency. Both phenotypical properties were correlated with lower Mg-chelatase activity. Unexpectedly, less protoporphy rin IX and heme accumulated, and a decrease in li-aminolevulinate (ALA)-syn thesizing capacity and ALA dehydratase activity paralleled the progressive reduction in Mg-chelatase activity in the transformants compared with contr ol plants. The reduced activities of the early enzymatic steps corresponded with lower levels of transcripts encoding glutamyl-tRNA reductase and ALA- dehydratase. The decreased expression and activities of early enzymes in th e pathway could be explained by a feedback-controlled mechanism in response to lower Mg-chelatase activity. We discuss intercompartmental signaling th at synchronizes the activities of the first steps in tetrapyrrolic metaboli sm with the late steps for the synthesis of end products.