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 mode l 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 pl ant sucrose synthases, in particular to the nodule-enhanced sucrose synthas es from pea and broad bean. Northern analysis revealed that the correspondi ng gene shows a ten-fold higher expression level in root nodules than in un infected root, stem and leaf tissues. SucS protein was detected in root nod ules from a variety of legumes, including M. truncatula. Whereas only one S ucS isoform was detectable in root nodules, an additional sucrose synthase of slightly larger molecular weight was present in uninfected root, stem an d newer 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 singl e-copy gene. An analysis of a genomic MtSucS1 sequence revealed that the ge ne consists of 14 exons with the start codon being located on exon II. As i s 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 ma pped and putative regulatory elements in the MtSucS1 promoter were identifi ed.