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Two Distinct Uricase II (Nodulin 35) Genes Are Differentially Expressed in Soybean Plants

Nodule-specific uricase (uricase II) is a homotetramer of a 33-kDa polypeptide, nodulin 35, and plays a key role in the assimilation of nitrogen fixed by microsymbionts in most legumes that have determinate nodules. We have isolated two distinct genes, UR2 and UR9, that encode for nodulin 35 from a soybean genomic library. Their corresponding cDNAs were also isolated from a nodule cDNA library. UR2 and UR9 both encode for 309 amino acid proteins with 12 amino acid differences. The expression of these two genes in various organs of soybean was examined by reverse transcription-polymerase chain reaction with primers specific to each cDNA sequences. Expression of UR9 was almost specific in root nodules, although it was expressed in roots, primary leaves, and developing seed at very low levels. In contrast, the UR2 transcripts were present in almost all plant organs at low levels, but no enhancement of the expression was observed in nodules. Thus, UR9 behaves as a nodulin gene, whereas UR2 is a nonsymbiotic uricase II gene. The sequences of their potential promoter regions share high homology within regions up to about 400 bp upstream from the translation initiation sites. These results suggest that symbiotic and nonsymbiotic uricase II genes diverged by gene duplication and that relatively small alterations in the promoter sequence enable the nodule-specific expression.