THE SUCROSE SYNTHASE GENE IS PREDOMINANTLY EXPRESSED IN THE ROOT-NODULE TISSUE OF VICIA-FABA

To analyze nodule-specific gene expression in broadbean, we have isolated and sequenced sucrose synthase (SUCS) cDNAs from a broadbean nodule-specific cDNA library. The most 5' sequences identified from these partial cDNAs were used as a molecular probe to isolate a full-length sucrose synthase transcript sequence from a cDNA library derived from broadbean nodule mRNA. This cDNA (YfSUCS) contained a reading frame of 2,418 bp, coding for a protein of 806 amino acids with a deduced molecular weight of 92.5 kDa. The DNA as well as the deduced amino acid sequence displayed substantial homologies (68-95%) to other plant SUCS sequences. Northern and RNA dot blot experiments demonstrated that this gene is strongly expressed in the broadbean nodule tissue. An at least 10-fold lower VfSUCS expression could be detected in the uninfected root, hypocotyl, stem, and flower tissues of broadbean, whereas only traces of VfSUCS transcripts were recognizable in the broadbean leaf tissues. VfSUCS transcripts could not be detected in mature seeds of broadbean. Because of this significantly nodule-amplified type of expression, we refer to YfSUCS as a nodulin gene and propose to designate it YfNOD93 (Nuf-93) for the sucrose synthase enzyme).