<p> Bacterial sulphur metabolism depends on the iron-containing porphinoid sirohaem. CysG is a multi-functional enzyme with S-adenosyl-L-methionine (SAM)-dependent bismethyltransferase, dehydrogenase and ferrochelatase activities. CysG synthesizes sirohaem from uroporphyrinogen III via reactions which encompass two branchpoint intermediates in tetrapyrrole biosynthesis, diverting flux first from protoporphyrin IX biosynthesis and then from cobalamin (vitamin B12) biosynthesis. CysG is a dimer. Its dimerisation region is 74 residues long, and acts to hold the two structurally similar protomers held together asymmetrically through a number of salt-bridges across complementary residues within the dimerisation region [<cite idref="PUB00029889"/>]. CysG dimerisation produces a series of active sites, accounting for CysG's multi-functionality, catalysing four diverse reactions:</p><p> <ul> <li>Two SAM-dependent methylations</li><li>NAD+-dependent tetrapyrrole dehydrogenation</li><li>Metal chelation</li> </ul> </p><p>This group represent a subfamily of CysG N-terminal region-related sequences. All sequences in the seed alignment for this model are N-terminal regions of known or predicted sirohaem synthases. The C-terminal region of each is uroporphyrin-III C-methyltransferase (<db_xref db="EC" dbkey="2.1.1.107"/>), which catalyses the first step committed to the biosynthesis of either sirohaem or cobalamin (vitamin B12) rather than protohaem (haem). Functionally these sequences complete the process of oxidation and iron insertion to yield sirohaem. Sirohaem is a cofactor for nitrite and sulphite reductases, so sirohaem synthase is CysG of cysteine biosynthesis in some organisms. </p>