HETEROLOGOUS EXPRESSION OF THE BCHM GENE-PRODUCT FROM RHODOBACTER-CAPSULATUS AND DEMONSTRATION THAT IT ENCODES S-ADENOSYL-L-METHIONINE-MG-PROTOPORPHYRIN-IX METHYLTRANSFERASE

The bacteriochlorophyll biosynthesis gene, bchM, from Rhodobacter capsulatus was previously believed to code for a polypeptide involved in formation of the cyclopentone ring of protochlorophyllide from Mg-protoporphyrin IX monomethyl ester. In this study, R. capsulatus bchM was expressed in Escherichia coli and the gene product was subsequently demonstrated by enzymatic analysis to catalyze methylation of Mg-protoporphyrin IX to form Mg-protoporphyrin IX monomethyl ester. Activity required the substrates Mg-protoporphyrin IX and S-adenosyl-L-methionine. C-14-labeled product was formed in incubations containing C-13-methyl-labeled S-adenosyl-L-methionine. On the basis of these and previous results, we also conclude that the bchH gene, which was previously reported to code for Mg-protoporphyrin IX methyltransferase, is most likely involved in the Mg chelation step.