Molecular cloning and sequence variation of UDP-glucose pyrophosphorylase cDNAs from potatoes sensitive and resistant to cold sweetening.

RT-PCR was used to isolate seven cDNAs encoding uridine diphosphate-glucose pyrophosphorylase (UGPase) from six potato cultivars that differed markedly in their ability to sweeten in cold storage (2-4 degrees C). These sequences were compared to two potato UGPase-cDNAs previously published. All cDNAs were highly conserved (97.6-99.9%) and coded for polypeptides with 477 amino acids. The cDNAs could be placed into two sequence classes depending on whether they contained a BamH1 site at nucleotide positions 1315-1320. The presence of the BamH1 site (substitution of a C for a T at bp position 1320) did not lead to a change of an amino acid in the mature protein. There were 27 nucleotide polymorphisms that co-segregated along with the BamH1 site, five of which led to an amino acid change (i.e., bp positions (5) Thr for Ala; (30) Glu for Asp; (82) Lys for Asn; (445) Lys for Glu; and (450) Val for Ile). All of the encoded polypeptides contained the five highly conserved lysine residues located at positions 263, 329, 367, 409 and 410 that have been demonstrated necessary for catalytic activity of UGPase. All polypeptides had putative glycosylation sites at amino acid positions 168 (NQS) and 307 (NLS). The Ser at position 420 provided a putative site for phosphorylation as well as a binding motif for 14-3-3 proteins.