Clusters of genes encoding enzymes for tetrapyrrole biosynthesis were cloned from and The sequences of all genes found, and of a 6·3 kbp gene cluster from were determined. The structure of the gene clusters was compared to that of other Gram-positive bacteria. The and species have a conserved organization of the genes required for biosynthesis of uroporphrinogen III (Urolll) from glutamyl-tRNA. In the genes for Urolll synthesis are also closely linked but their organization is different: there is no gene and the gene cluster also contains genes, and - encoding the enzymes required for synthesis of sirohaem from Urolll. contains genes for three proteins, NasF, YlnD and YlnF, with sequence similarity to coli CysG, which is a multi-functional protein catalysing sirohaem synthesis from Urolll. It is shown that YlnF is required for sirohaem synthesis and probably catalyses the precorrin-2 to sirohaem conversion. YlnD probably catalyses precorrin-2 synthesis from Uroll and NasF seems to be specific for nitrite reduction.


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