title: MosA, a protein implicated in rhizopine biosynthesis in Sinorhizobium meliloti L5-30, is a dihydrodipicolinate synthase.
abstract: MosA is a gene product encoded on a pSym megaplasmid of Sinorhizobium meliloti L5-30. The gene is part of an operon reported to be essential for the synthesis of the rhizopine 3-O-methyl-scyllo-inosamine. MosA has been assigned the function of an O-methyltransferase. However, the reported sequence of this protein is very much like that of dihydrodipicolinate synthase (DHDPS), except for a 40 amino acid residue C-terminal domain. This similarity contradicts accepted ideas regarding structure-function relationships of enzymes. We have cloned and overexpressed the recombinant gene in Escherichia coli, and discovered that the reported sequence contains an error resulting in a frame-shift. The correct sequence contains a new stop codon, truncating the C-terminal 41 amino acid residues of the reported sequence. The expressed protein, bearing an N-terminal polyhistidine tag, catalyzes the condensation of pyruvate and aspartate beta-semialdehyde efficiently, suggesting that this activity is not a side-reaction, but an activity for which this enzyme has evolved. Electro-spray mass spectrometry experiments and inhibition by L-lysine are consistent with the enzyme being a DHDPS. E.coli AT997, a mutant host normally requiring exogenous diaminopimelate for growth, could be complemented by transformation with a plasmid bearing the gene encoding MosA. A role for this enzyme in rhizopine synthesis cannot be ruled out, but is called into question.
authors.affiliation: Department of Biochemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Sask., Canada S7N 5C9.
authors.forename: Pui Hang
authors.initials: PH
authors.lastname: Tam
authors.forename: Christopher P
authors.initials: CP
authors.lastname: Phenix
authors.forename: David R J
authors.initials: DR
authors.lastname: Palmer
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry*; D019755:Carbon-Carbon Lyases / Q000737:chemistry*; D003001:Cloning, Molecular; D004926:Escherichia coli; D005816:Genetic Complementation Test; D006836:Hydro-Lyases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D007294:Inositol / Q000031:analogs & derivatives* / Q000096:biosynthesis*; D008239:Lysine / Q000494:pharmacology*; D008969:Molecular Sequence Data; D009876:Operon; D017386:Sequence Homology, Amino Acid; D016962:Sinorhizobium meliloti / Q000201:enzymology* / Q000235:genetics; D021241:Spectrometry, Mass, Electrospray Ionization
chemical_list: D001426:Bacterial Proteins; C082577:rhizopine; D007294:Inositol; C104866:MosA protein, Rhizobium meliloti; D019755:Carbon-Carbon Lyases; D006836:Hydro-Lyases; C020040:4-hydroxy-tetrahydrodipicolinate synthase; D008239:Lysine