Summary auto-generated
This study investigates iron acquisition mechanisms in Rhizobium leguminosarum, a bacterium that forms nitrogen-fixing symbioses with legume plants. The researchers demonstrated that mutations in the cycHJKL operon, which encodes proteins involved in cytochrome c maturation, cause pleiotropic defects including loss of high-affinity iron uptake, inability to produce or export siderophores (iron-chelating compounds), and accumulation of protoporphyrin IX. The previously characterized Pop116 mutant was shown to harbor a mutation in cycK. The authors identified three genes upstream of cycHJKL: lipA (encoding a predicted outer-membrane lipoprotein) and feuP and feuQ (encoding two-component transcriptional regulators homologous to PhoP and PhoQ). A feuQ mutation abolished high-affinity iron uptake despite normal siderophore production, indicating FeuQ functions downstream of siderophore synthesis in iron transport. However, feuQ mutations did not affect transcription of cycHJKL or lipA. The study proposes that cycHJKL products are required for functional iron uptake systems independent of their role in cytochrome c biogenesis.
Key findings
- The Pop116 mutant carries a cycK mutation causing pleiotropic defects: loss of high-affinity iron uptake, failed siderophore production/export, and protoporphyrin IX accumulation
- The feuPQ genes encode two-component regulatory proteins homologous to PhoP-PhoQ systems, with FeuQ functioning as a sensor histidine kinase and FeuP as a response regulator
- FeuQ is required for high-affinity iron uptake independent of siderophore biosynthesis, suggesting its role in iron-siderophore transport rather than synthesis regulation
- FeuQ mutations do not affect transcription of cycHJKL or lipA operons, indicating post-transcriptional regulation of iron uptake
- CycHJKL mutations cause iron acquisition defects through mechanisms beyond their known role in cytochrome c maturation
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Abstract
The cycHJKL operon of Rhizobium leguminosarum has previously been shown to be involved in the maturation of cytochrome c, possibly by its involvement in the covalent attachment of haem to the apoprotein. Mutations in the cycHJKL genes abolish symbiotic nitrogen fixation. Here, we show that cyc 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 cycH is a gene, lipA, which is predicted to encode an outer-membrane lipoprotein. Further upstream of lipA, there are two other genes, whose products are similar in sequence to the widespread family of two-component transcriptional regulators. These two genes, feuP and feuQ, did not affect the transcription of lipA, or of the cycHJKL operon. However, a mutation in feuQ also led to the loss of the high affinity iron uptake system, although siderophores were still produced.