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Analysis of the Rhizobium leguminosarum siderophore-uptake gene fhuA: differential expression in free-living bacteria and nitrogen-fixing bacteroids and distribution of an fhuA pseudogene in different strains

School of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, UK¹
John Innes Centre, Norwich Research Park, Colney Lane, Norwich NR4 7UH, UK²

Author for correspondence: Andrew W. B. Johnston. Tel: +44 1603 592264. Fax: +44 1603 592250. e-mail: a.johnston{at}uea.ac.uk

A mutation was isolated in the Rhizobium leguminosarum gene fhuA, which appears to specify the outer-membrane receptor for the siderophore vicibactin. The mutant was defective in iron uptake and accumulated the siderophore vicibactin in the extracellular medium. Expression of fhuA was regulated by Fe³⁺, transcription being higher in iron-depleted cells. Transcription of fhuA was independent of a functional copy of rpoI, a neighbouring gene that specifies a putative ECF factor of RNA polymerase and which is involved in siderophore production in Rhizobium. Mutations in fhuA did not detectably affect symbiotic N₂ fixation on peas. An fhuA::gus fusion was expressed by bacteria in the meristematic zone of pea nodules but not in mature bacteroids. Some other strains of R. leguminosarum also contain a pseudogene version of fhuA. The sequences of some of these and the ‘real’ fhuA genes were determined.

Keywords: ECF factor, fhu genes, iron-mediated regulation, pseudogene, rhizobia, siderophores

Abbreviations: CAS, chrome azurol sulphonate; ECF, extracytoplasmic factor; NTA, nitrilotriacetate; X-Gluc, 5-bromo-4-chloro-3-indolyl ß-D-glucuronide

The GenBank/EMBL/DDBJ accession number for the sequence determined in this work is AJ238208.

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