ANAEROBIC PROTOPORPHYRIN BIOSYNTHESIS DOES NOT REQUIRE INCORPORATION OF METHYL-GROUPS FROM METHIONINE

It was recently reported (H. Akutsu, J,-S, Park, and S. Sane, J, Am, C hem, Soc. 115:12185-12186, 1993) that in the strict anaerobe Desulfovi brio vulgaris methyl groups from exogenous L-methionine are incorporat ed specifically into the 1 and 3 positions (Fischer numbering system) on the heme groups of cytochrome c(3). It was suggested that under ana erobic conditions, protoporphyrin IX biosynthesis proceeds via a novel pathway that does not involve coproporphyrinogen III as a precursor h ut instead may use precorrin-2 (1,3-dimethyluroporphyrinogen III), a s iroheme and vitamin B-12 precursor which is known to be derived from u roporphyrinogen III via methyl transfer from S-adenosyl-L-methionine. We have critically tested this hypothesis by examining the production of protoporphyrin IX-based tetrapyrroles in the presence of exogenous [C-14]methyl-L-methionine under anaerobic conditions in a strict anaer obe (Chlorobium vibrioforme) and a facultative anaerobe (Rhodobacter c apsulatus). In both organisms, C-14 was incorporated into the bacterio chlorophyll precursor, Mg-protoporphyrin IX monomethyl ester, However, most of the label was lost upon base hydrolysis of this compound to y ield Mg-protoporphyrin IX, These results indicate that although the ad ministered [C-14]methyl-L-methionine was taken up, converted into S-ad enosyl-L-methionine, and used for methyl transfer reactions, including methylation of the 6-propionate of Mg-protoporphyrin IX, methyl group s were not transferred to the porphyrin nucleus of Mg-protoporphyrin I X, In other experiments, a cysG strain of Salmonella typhimurium, whic h cannot synthesize precorrin-2 because the gene encoding the enzyme t hat catalyzes methylation of uroporphyrinogen III at positions 1 and 3 is disrupted, was capable of heme dependent anaerobic nitrate respira tion and growth on the nonfermentable substrate glycerol, indicating t hat anaerobic biosynthesis of protoporphyrin IX-based hemes does not r equire the ability to methylate uroporphyrinogen III, Together, these results indicate that incorporation of L-methionine-derived methyl gro ups into porphyrins or their precursors is not generally necessary for the anaerobic biosynthesis of protoporphyrin IX-based tetrapyrroles.