Nucleotide Sequence of S-Adenosyl-l-Methionine: Magnesium Protoporphyrin Methyltransferase from Rhodobacter capsulatus.

It is generally believed that Chl and bacteriochlorophyll biosynthesis are evolutionarily related. This supposition is supported by the observation that the biosynthetic pathway for Chl a involves intermediates that are common with bacteriochlorophyll a biosynthesis (reviewed in ref. 7) and by the observation that light-independent reduction of protochlorophyllide to Chlide involves an enzyme complex that is highly conserved between purple photosynthetic bacteria and plants (J. Suzuki and C. Bauer, unpublished data). Thus, photosynthetic bacteria offer a good model system for studying enzymes involved in Chl biosynthesis. The bacterium Rhodobacter capsulatus has the capability to undergo rapid growth under nonphotosynthetic (heterotrophic) conditions and is, therefore, particularly amenable to a genetic analysis ofpigment biosynthesis. Numerous mutants deficient in bacteriochlorophyll biosynthesis have been isolated that accumulate different intermediates in the Mg-tetrapyrrole biosynthetic pathway (9, 10). Of particular interest are mutations in the bchH gene which are reported to lack Sadenosyl-L-methionine: magnesium protoporphyrin methyltransferase activity which catalyzes the transfer of a methyl group from S-adenosyl-L-methionine to the sixth propyl group of Mg-protoporphyrin IX (6). The sequence of bchH (Table I, Fig. 1) encodes a large polypeptide of 1195 residues. The predicted protein has no apparent membrane-spanning region and its mean hydrophobicity (-0.116) is typical of soluble proteins (4). It has no readily identifiable homolog in the Genbank data base. S-adenosyl-L-methionine: magnesium protoporphyrin methyltransferase is interesting from a biochemical point of