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

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 rpol a neighbouring gene that specifies a putative ECF sigma factor of RNA polymerase and which is involved in siderophore production in Rhizobium, Mutations in fhuA did not detectably affect symbiotic N-2 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.