cl00304

S-AdoMet dependent tetrapyrrole methylases; This family uses S-AdoMet (S-adenosyl-L-methionine or SAM) in the methylation of diverse substrates. Most members catalyze various methylation steps in cobalamin (vitamin B12) biosynthesis. There are two distinct cobalamin biosynthetic pathways in bacteria. The aerobic pathway requires oxygen, and cobalt is inserted late in the pathway; the anaerobic pathway does not require oxygen, and cobalt insertion is the first committed step towards cobalamin synthesis. The enzymes involved in the aerobic pathway are prefixed Cob and those of the anaerobic pathway Cbi. Most of the enzymes are shared by both pathways and a few enzymes are pathway-specific. Diphthine synthase and Ribosomal RNA small subunit methyltransferase I (RsmI) are two superfamily members that are not involved in cobalamin biosynthesis. Diphthine synthase participates in the posttranslational modification of a specific histidine residue in elongation factor 2 (EF-2) of eukaryotes and archaea to diphthamide. RsmI catalyzes the 2-O-methylation of the ribose of cytidine 1402 (C1402) in 16S rRNA using S-adenosylmethionine (Ado-Met) as the methyl donor.