| wo classes of sucrose synthase genes were identified in potato. The T-class gene, Stsst16, and the V-class gene, Stssv65, are both split by 13 introns including a long leader intron. The two genes are 88% and 96% identical at the nucleotide and amino acid level, respectively, in the coding regions, but diverge outside the coding regions except for a few short conserved sequences. RNA gel blot analysis showed that T-class genes were expressed at much higher levels in tubers than in stems or roots. T-class genes were normally not detectable in leaves, but could be induced to be expressed at high levels by sucrose. In contrast, V-class genes were expressed at higher levels in roots and stems and were not sucrose-inducible. These results, combined with the GUS expression patterns observed in transgenic potato plants driven by regulatory sequences from either Stsst16 or Stssv65, suggest that a major role of the T-class genes is to supply substrates for starch synthesis in tubers, while the V-class genes serve to supply energy for phloem loading and substrates for general cellular respiration and metabolism.;By examining expression of a series of deletion and substitution constructs in transgenic plants, sequences critical for proper expression of the two classes of genes were determined. For the T-class gene, Stsst16, both high-level expression in tubers and sucrose inducibility required interaction of the 5;For the V-class gene,... |
