Deduced Amino Acid Sequence of a Plant cDNA Containing a Leucine Zipper Motif

We screened a tomato (Lycopersicon esculentum L.) cDNA library (1.3 x 106 plaque-forming units [7]) with a polymerase chain reaction fragment that was thought to be a specific probe for a shikimate dehydrogenase, and we isolated 11 independent clones (Table I). The cDNA inserts were subcloned into pSK+ (Stratagene) and sequenced from the 3' end. All 11 clones contained the same sequence, and the clone with the largest insert (called sgutl) was sequenced completely (Fig. 1). The cDNA without the polyadenylation tail has a length of 2096 bp and contains an open reading frame from its 5' end to bp position 1902. The first ATG triplet at position 10 is situated in this open reading frame, and the sequence around it (TATCCATGGCT) shows homology to the consensus sequence around plant initiation codons (TAACAATGGCT [3]), which suggests that the cDNA is a full-length clone encoding a polypeptide of 631 amino acids. The function of this protein is unknown, and a sequence comparison to the EMBL data base revealed no similarity to known proteins. Searching for protein motifs using the University of Wisconsin Genetics Computer Group program package (Motifs, version 7.0) led to the identification of a potential leucine zipper. A certain class of DNA-binding protein contains leucine zippers that are responsible for dimerization of transcription factors by the formation of coiledcoil structures (2, 4). The DNA-binding domain of these proteins is normally a basic domain that is in close proximity to the leucine zipper (1). However, no such basic domain could be identified in the deduced sgutl sequence. It contains a very acidic region of six glutamic acid residues followed by one valine and seven aspartic acid residues (Fig. 1; amino acid position 157-170), for which no function is known. The sgutl peptide contains an unusually high amount of charged residues (Asp + Glu = 16%; His + Lys + Arg = 15%).