Molecular mechanism of symbiotic specific gene expression

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The profiles of ureide (allantoin and allantoic acid) accumulation under different nitrogen nutrient conditions were surveyed in various tissues of alfalfa, Medicago sativa. The ratios of ureide to asparagine in xylem saps of two amide type legumes, Medicago sativa and Lotus corniculatus were 14 times less than that of a typical ureide type legume, Glycine max. when they are growing with nitrogen-free nutrient solution.Two cDNA clones, MsU2 and MsU9 encoding uricase (EC 1.7.3.3., Nodulin-35) were isolated from a cDNA library prepared from nodule tissues of alfalfa, Medicago sativa, plants. Both MsU2 and MsU9 encoded 3 08 amino acid polypeptides with a difference of 5 amino acids, and the deduced amino acid sequences shared 980/0 homology. Between these two cDNA clones and uricase genes of soybean which were designated as Nod-35s, more than 800/0 identity was observed in nucleotides and deduced amino acid sequences, suggesting that these MsU2 and MsU9 are homologs of Nod-35.Using the reverse transcription-PCR technique, we detected the transcripts of these two genes in almost all tissues of alfalfa. The operation of uricase genes was confirmed by the presence of ureide in the xylem sap and uricase activity in the nodules. In situ hybridization analysis revealed that MsU2 and MsU9 were expressed only in uninfected cells of the infected zone of the nodule tissue. The cell specific-expression of the two uricase genes was observed in an identical manner to that of Nod-35 in soybean nodules.