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dpp Genes of Rhizobium leguminosarum Specify Uptake of δ-Aminolevulinic Acid

An operon with homology to the dppABCDF genes required to transport dipeptides in bacteria was identified in the N₂-fixing symbiont, Rhizobium leguminosarum. As in other bacteria, dpp mutants were severely affected in the import of δ-aminolevulinic acid (ALA), a heme precursor. ALA uptake was antagonized by adding dipeptides, indicating that these two classes of molecule share the same transporter. Mutations in dppABCDF did not affect symbiotic N₂ fixation on peas, suggesting that the ALA needed for heme synthesis is not supplied by the plant or that another uptake system functions in the bacteroids. The dppABCDF operon of R. leguminosarum resembles that in other bacteria, with a gap between dppA and dppB containing inverted repeats that may stabilize mRNA and may explain why transcription of dppA alone was higher than that of dppBCDF. The dppABCDF promoter was mapped and is most likely recognized by σ⁷⁰.