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. IMAGES:

编码植物生物合成型苏氨酸脱氨酶（threonine deaminase, Td; EC 4.2.1.16）的基因，因在番茄花中出现异常上调表达而被成功克隆。番茄的Td基因编码一条含595个氨基酸残基的多肽，其前80个氨基酸构成一个推测的双结构域转运肽，该转运肽在第51位氨基酸位点处被切割。将该氨基酸序列与酵母和细菌中的同源酶进行比对后发现，其关键催化区域几乎完全保守，整体序列相似性超过40%。该Td基因在番茄基因组中为单拷贝基因，其编码区被8个内含子打断。该基因在萼片中的表达量较叶片或根组织高50倍以上，在花的其余组织中的表达量则较叶片或根高500倍以上，不过其高表达严格局限于花器官的薄壁细胞中。在幼嫩番茄叶片中，定位于叶绿体的该酶几乎仅存在于表皮下的海绵叶肉细胞中。图片：