cDNA and gene sequences of wheat chloroplast sedoheptulose‐1,7‐bisphosphatase reveal homology with fructose‐1,6‐bisphosphatases

The nucleotide sequence encoding the chloroplast enzyme, sedoheptulose‐l,7‐bisphosphatase [Sed(l,7)P2ase], was obtained from wheat cDNA and genomic clones. The transcribed region of the Sed(1,7)P2ase gene has eight exons (72–507 bp) and seven introns (85–626 bp) and encodes a precursor polypeptide of 393 amino acids. Comparison of the deduced amino acid sequence of Sed(1,7)P2ase with those of fructose‐1,6‐bisphosphatase[Fru(1,6)P2ase] enzymes from a variety of sources reveals 19% identity, rising to 42% if conservative changes are considered. Most importantly, the amino acid residues which form the active site of Fru(l,6)P2ase are highly conserved in the Sed(1,7)P2ase molecule, indicating a common catalytic mechanism. Interestingly, although the activities of both Sed(1,7)P2ase and chloroplast Fru(1,6)P2ase are modulated by light via the thioredoxin system, the amino acid sequence motif identified as having a role in this regulation in chloroplast Fru(1,6)P2ase is not found in the Sed(l,7)P2ase enzyme.