Genomic organization and expression properties of the MtSucS1 gene, which encodes a nodule-enhanced sucrose synthase in the model legume Medicago truncatula.

We have isolated and sequenced a sucrose synthase (SucS) cDNA from the model legume Medicago truncatula. This cDNA (MtSucS1) contains an ORF of 2418 bp, coding for a protein of 805 amino acids with a molecular mass of 92.29 kDa. The deduced amino acid sequence shows significant homology to other plant sucrose synthases, in particular to the nodule-enhanced sucrose synthases from pea and broad bean. Northern analysis revealed that the corresponding gene shows a ten-fold higher expression level in root nodules than in uninfected root, stem and leaf tissues. SucS protein was detected in root nodules from a variety of legumes, including M. truncatula. Whereas only one SucS isoform was detectable in root nodules, an additional sucrose synthase of slightly larger molecular weight was present in uninfected root, stem and flower tissues of M. truncatula. From our expression and sequence data we infer that the MtSucS1 gene encodes a nodule-enhanced sucrose synthase in M. truncatula. Southern hybridization data indicate that MtSucS1 is a single-copy gene. An analysis of a genomic MtSucS1 sequence revealed that the gene consists of 14 exons with the start codon being located on exon II. As is common for SucS genes, the MtSucS1 gene contains a large intron of 747 bp in the 5' untranslated region. The transcriptional start of MtSucS1 was mapped and putative regulatory elements in the MtSucS1 promoter were identified.