2 DISTINCT URICASE-II (NODULIN-35) GENES ARE DIFFERENTIALLY EXPRESSEDIN SOYBEAN PLANTS

Nodule-specific uricase (uricase II) is a homotetramer of a 33-kDa pol ypeptide, nodulin 35, and plays a key role in the assimilation of nitr ogen fixed by microsymbionts in most legumes that have determinate nod ules, We have isolated two distinct genes, UR2 and UR9, that encode fo r nodulin 35 from a soybean genomic library, Their corresponding cDNAs were also isolated from a nodule cDNA library, UR2 and UR9 both encod e for 309 amino acid proteins with 12 amino acid differences. The expr ession 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 deve loping seed at very low levels, In contrast, the UR2 transcripts were present in almost all plant organs at low levels, but no enhancement o f the expression was observed in nodules, Thus, UR9 behaves as a nodul in 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, Th ese results suggest that symbiotic and nonsymbiotic uricase II genes d iverged by gene duplication and that relatively small alterations in t he promoter sequence enable the nodule-specific expression.