Biosynthetic threonine deaminase gene of tomato: isolation, structure, and upregulation in floral organs.

The gene encoding the plant biosynthetic threonine deaminase (Td; EC 4.2.1.16) has been cloned as a result of its unusual upregulation in tomato flowers. The Td gene of tomato encodes a polypeptide of 595 residues, the first 80 of which comprise a putative two-domain transit peptide cleaved at position 51. Comparison of the amino acid sequence with the corresponding enzymes from yeast and bacteria reveals a near identity of the important catalytic regions and greater than 40% overall similarity. The Td gene is unique in the tomato genome and its coding region is interrupted by eight introns. Its expression is greater than 50-fold higher in sepals and greater than 500-fold higher in the rest of the flower than in leaves or roots. Its overexpression, however, is strictly confined to the parenchymal cells of the floral organs. In young tomato leaves, the chloroplast-bound enzyme is found almost exclusively in the subepidermal spongy mesophyll cells.