The synthesis of the modified tetrapyrrole known asd1haem requires sev-eral dedicated proteins which are coded for by a set of genes that are often found adjacent to the structural gene, nirS, for cytochrome cd1 nitrite reductase. NirE, the product of the first gene in the nirbiogenesis operon, was anticipated to catalyse the conversion of uroporphyrinogen III into precorrin-2