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 caps ulatus was previously believed to code for a polypeptide involved in f ormation of the cyclopentone ring of protochlorophyllide from Mg-proto porphyrin IX monomethyl ester. In this study, R. capsulatus bchM was e xpressed in Escherichia coli and the gene product was subsequently dem onstrated by enzymatic analysis to catalyze methylation of Mg-protopor phyrin IX to form Mg-protoporphyrin IX monomethyl ester. Activity requ ired 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 re sults, we also conclude that the bchH gene, which was previously repor ted to code for Mg-protoporphyrin IX methyltransferase, is most likely involved in the Mg chelation step.