A NOVEL ZINC-FINGER PROTEIN ENCODED BY A COUCH POTATO HOMOLOG FROM SOLANUM-TUBEROSUM ENABLES A SUCROSE TRANSPORT DEFICIENT YEAST-STRAIN TO GROW ON SUCROSE

A yeast strain deficient in secreted invertase but expressing a cytopl asmic sucrose synthase has been used to select for potato genes that e nable growth on sucrose as the sole carbon source by suppressing the s ucrose uptake deficiency. Besides the already known sucrose transporte r gene (StSUT1), ten different suppressor clones were identified and c haracterized. One of these cDNAs (PCP1) enabled efficient growth of th e mutant yeast strain and mediated uptake of radiolabelled sucrose. Th e cDNA encodes a protein of 509 amino acids which is highly hydrophili c and thus does not seem to represent a transporter. Sequence comparis ons show that the protein contains zinc finger motifs and shares weak homologies with the Drosophila couch potato gene, which serves as a tr anscriptional regulator, indicating that PCP1 activates a silent endog enous sucrose uptake system. The other suppressor clones encode either putative transcriptional regulators, protein kinases or enzymes invol ved in thiamine biosynthesis, ferredoxin reduction or glutamyl tRNA re duction and suppress the phenotype by unknown mechanisms.