Factor S3, an unusual metabolite of Propionibacterium shermanii, is bi
osynthesized from porphobilinogen (PBG) by three enzymes, PBG deaminas
e, uro'gen III methyltransferase (M1), and the Salmonella typhimurium
cbiF gene product, isolated from recombinant strains of Escherichia co
li. PBG deaminase affords uro'gen I, and M1 performs the methylation o
f three beta-pyrrolic carbons of uro'gen I to form 2,7,12-trimethylpyr
rocorphin I. The cbiF gene product then methylates one alpha-pyrrolic
carbon (C-16). Nonenzymatic insertion of zinc into the macrocycle form
s the tetramethylated zinc corphinoid factor S3.