The Rhizobium leguminosarum tonB gene is required for the uptake of siderophore and haem as sources of iron

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Abstract
In the N2‐fixing bacterium Rhizobium leguminosarum, mutations in a homologue of tonB (tonBRl) block the import of vicibactin and haem as iron sources in free‐living bacteria. TonBRl mutants were normal for growth with ferric dicitrate and slightly reduced for growth with haemoglobin as sole iron sources. The deduced TonBRl product is larger than that of (for example) Escherichia coli, on account of an extended N‐terminal domain. Transcription of tonBRl was enhanced in low‐Fe growth conditions; this was not controlled by Fur, nor RpoI, an Fe‐regulated extracytoplasmic σ factor. Upstream of tonBRl and transcribed divergently is an operon, hmuPSTUV, whose products are homologous to ABC transporters involved in haem uptake in pathogenic bacteria. Expression of hmuPSTUV was enhanced in low‐Fe conditions, and hmu mutants show slightly diminished growth on haem as sole Fe source, suggesting that there is more than one system for the uptake of this molecule. hmuPSTUV expression appears to be from three closely linked promoters. Downstream of hmuPSTUV, a gene that may encode an extracytoplasmic σ factor was identified, but this gene, rpoZ, did not affect the transcription of tonBRl or hmuPSTUV. Mutations in tonBRl, hmu genes and rpoZ did not affect symbiotic N2 fixation in peas.