The biosynthesis of the tetrapyrrole framework has been investigated in the sulfate-reducing bacterium Desulfovibrio vulgaris Hildenborough by characterization of the enzymes required for the transformation of aminolaevulinic acid into sirohydrochlorin. PBG (porphobilinogen) synthase (HemB) was found to be at zinc-dependent enzyme that exists in its native state as a homohexamer. PBG deaminase (HemC) was shown to contain the dipyrromethane cofactor. Uroporphyrinogen III synthase is found fused with a uroporphyrinogen III methyltransferase (HemD-CobA). Both activities Could be demonstrated in this amalgamated protein and the individual enzyme activities were separated by dissecting the relevant gene to allow the production of two distinct proteins. A gene annotated in the genome as a bifunctional precorrin-2 dehydrogenase/sirohydrochlorin ferrochelatase was in I act shown to act only as a dehydrogenase and is simply capable of synthesizing sirohydrochlorin rather than sirohaem. Genome analysis also reveals a lack of any uroporphyrinogen Ill decarboxylase, all enzyme necessary for the classical route to heam synthesis. However, tile genome does encode soiree predicted haem d(1) biosynthetic enzymes even though tile bacterium does not contain the cd(1) nitriate reductase, We suggest that sirohydrochlorin acts as a substrate for heam synthesis using a novel pathway that involves homologues of the d(1) biogenesis system. This explains why the uroporphyrinogen III synthase is found fused with tile methyltransferase, bypassing tile need for uroporphyrinogen III decarboxylase activity.