The operon of has previously been shown to be involved in the maturation of cytochrome , possibly by its involvement in the covalent attachment of haem to the apoprotein. Mutations in the genes abolish symbiotic nitrogen fixation. Here, we show that mutants are pleiotropically defective. They have lost a high affinity iron acquisition system due to their failure to make or to export siderophores. They also accumulate protoporphyrin IX, the immediate precursor of haem. A model to account for these phenotypes is presented. Immediately upstream of is a gene, , which is predicted to encode an outer-membrane lipoprotein. Further upstream of , there are two other genes, whose products are similar in sequence to the widespread family of two-component transcriptional regulators. These two genes, and , did not affect the transcription of , or of the operon. However, a mutation in also led to the loss of the high affinity iron uptake system, although siderophores were still produced.


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