Genomic organization of the cobalamin biosynthetic genes and regulatory B12-elements (yellow cloverleaf-type structures). Genes of the first part of the pathway, involved in the corrin ring synthesis are shown as yellow arrows, the genes required for the attachment of the aminopropanol arm and assembly of the nucleotide loop in vitamin B12 are in green. Cobalt transporters and chelatases used for the insertion of cobalt ions into the corrin ring are shown in pink and orange, respectively. ABC transport systems for vitamin B12 are shown in blue. See Figure 1 legend for abbreviations.