title: Bradyrhizobium japonicum delta-aminolevulinic acid dehydratase is essential for symbiosis with soybean and contains a novel metal-binding domain.
abstract: The Bradyrhizobium japonicum hemA gene product delta-aminolevulinic acid (ALA) synthase is not required for symbiosis of that bacterium with soybean. Hence, the essentiality of the subsequent heme synthesis enzyme, ALA dehydratase, was examined. The B. japonicum ALA dehydratase gene, termed hemB, was isolated and identified on the basis of its ability to confer hemin prototrophy and enzyme activity on an Escherichia coli hemB mutant, and it encoded a protein that was highly homologous to ALA dehydratases from diverse organisms. A novel metal-binding domain in the B. japonicum ALA dehydratase was identified that is a structural composite of the Mg(2+)-binding domain found in plant ALA dehydratases and the Zn(2+)-binding region of nonplant ALA dehydratases. Enzyme activity in dialyzed extracts of cells that overexpressed the hemB gene was reconstituted by the addition of Mg2+ but not by addition of Zn2+, indicating that the B. japonicum ALA dehydratase is similar to the plant enzymes with respect to its metal requirement. Unlike the B. japonicum hemA mutant, the hemB mutant strain KP32 elicited undeveloped nodules on soybean, indicated by the lack of nitrogen fixation activity and plant hemoglobin. We conclude that the hemB gene is required for nodule development and propose that B. japonicum ALA dehydratase is the first essential bacterial enzyme for B. japonicum heme synthesis in soybean root nodules. In addition, we postulate that ALA is the only heme intermediate that can be translocated from the plant to the endosymbiont to support bacterial heme synthesis in nodules.
authors.affiliation: Department of Biochemistry, State University of New York at Buffalo 14214.
authors.forename: S
authors.initials: S
authors.lastname: Chauhan
authors.forename: M R
authors.initials: MR
authors.lastname: O'Brian
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D003001:Cloning, Molecular; D004269:DNA, Bacterial / Q000302:isolation & purification / Q000378:metabolism; D004492:Edetic Acid / Q000494:pharmacology; D004926:Escherichia coli; D015870:Gene Expression; D005798:Genes, Bacterial; D005816:Genetic Complementation Test; D015698:Genomic Library; D006801:Humans; D007700:Kinetics; D008274:Magnesium / Q000378:metabolism / Q000494:pharmacology; D008969:Molecular Sequence Data; D010957:Plasmids; D000623:Porphobilinogen Synthase / Q000235:genetics* / Q000378:metabolism*; D011994:Recombinant Proteins / Q000494:pharmacology; D012230:Rhizobiaceae / Q000201:enzymology* / Q000235:genetics / Q000502:physiology*; D017386:Sequence Homology, Amino Acid; D013025:Glycine max / Q000502:physiology*; D013559:Symbiosis; D015032:Zinc / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D004269:DNA, Bacterial; D011994:Recombinant Proteins; D004492:Edetic Acid; D000623:Porphobilinogen Synthase; D008274:Magnesium; D015032:Zinc