Fur is not the global regulator of iron uptake genes in Rhizobium leguminosarum.

Rhizobium leguminosarum fur mutants were unaffected in Fe-dependent regulation of several operons that specify different Fe uptake systems, yet cloned R. leguminosarum fur partially corrected an Escherichia coli fur mutant and R. leguminosarum Fur protein bound to canonical fur boxes. The lack of a phenotype in fur mutants is not due to functional redundancy with Irr, another member of the Fur superfamily found in the rhizobia, since irr fur double mutants are also unaffected in Fe-responsive regulation of several operons involved in Fe uptake. Neither Irr nor Fur is needed for symbiotic N(2) fixation on peas. As in Bradyrhizobium japonicum, irr mutants accumulated protoporphyrin IX. R. leguminosarum irr is not regulated by Fur and its Irr protein lacks the motif needed for haem-dependent post-translational modification that occurs in B. japonicum Irr. The similarities and differences in the Fur superfamily in the rhizobia and other Gram-negative bacteria are discussed.