MAGNESIUM-PROTOPORPHYRIN CHELATASE OF RHODOBACTER-SPHAEROIDES - RECONSTITUTION OF ACTIVITY BY COMBINING THE PRODUCTS OF THE BCHH, BCHI, ANDBCHD GENES EXPRESSED IN ESCHERICHIA-COLI

Magnesium-protoporphyrin chelatase lies at the branch point of the hem e and (bacterio)chlorophyll biosynthetic pathways. In this work, the p hotosynthetic bacterium Rhodobacter sphaeroides has been used as a mod el system for the study of this reaction. The bchH and the bchI and -D genes from R. sphaeroides were expressed in Escherichia coli. When ce ll-free extracts from strains expressing BchH, BchI, and BchD were com bined, the mixture was able to catalyze the insertion of Mg into proto porphyrin IX in an ATP-dependent manner. This was possible only when a ll three genes were expressed. The bchH, -I, and -D gene products are therefore assigned to the Mg chelatase step in bacteriochlorophyll bio synthesis. The mechanism of the Mg chelation reaction acid the implica tions for chlorophyll biosynthesis in plants are discussed.