MosA, a protein implicated in rhizopine biosynthesis in Sinorhizobium meliloti L5-30, is a dihydrodipicolinate synthase.
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.
Tam
Pui Hang
PH
Department of Biochemistry, University of Saskatchewan, 110 Science Place, Saskatoon, Sask., Canada S7N 5C9.
Phenix
Christopher P
CP
Palmer
David R J
DR
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
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