Gregerson R G, Petrowski M, Larson R L, Gantt J S, Vance C P
US Department of Agriculture-Agricultural Research Service, Plant Science Research Unit, St. Paul 55108.
Mol Gen Genet. 1993 Oct;241(1-2):124-8. doi: 10.1007/BF00280209.
Aspartate aminotransferase (AAT) plays a key enzymatic role in the assimilation of symbiotically fixed nitrogen in legume root nodules. In alfalfa, two distinct genetic loci encode dimeric AAT enzymes: AAT1, which predominates in roots, and AAT2, which is expressed at high levels in nodules. Three allozymes of AAT2 (AAT2a, -2b and -2c), differing in net charge, result from the expression of two alleles, AAT2A and AAT2C, at this locus. Utilizing antiserum to alfalfa AAT2, we have previously isolated from an expression library one AAT2 cDNA clone. This clone was used as a hybridization probe to screen cDNA libraries for additional AAT2 cDNAs. Four different clones were obtained, two each that encode the AAT2a and AAT2c enzyme subunits. These two sets of cDNAs encode polypeptides that differ in net charge depending upon the amino acid at position 296 (valine or glutamic acid). Within each set of alleles, the two members differ from each other by the presence or absence of a 30 bp (ten amino acid) sequence. The presence or absence of this ten amino acid sequence has no effect on the size or charge of the mature AAT2 protein because it is located within the region encoding the protein's transit peptide, which is proteolytically removed upon transport into plastids. The data suggest that a deletion event has occurred independently in two AAT2 progenitor alleles, resulting in the four allelic cDNA variants observed. The deletion of this ten amino acid sequence does not appear to impair the normal maturation of the enzyme.
天冬氨酸转氨酶（AAT）在豆科植物根瘤中共生固定氮的同化过程中发挥关键的酶促作用。在苜蓿中，两个不同的基因座编码二聚体AAT酶：AAT1在根中占主导，AAT2在根瘤中高水平表达。AAT2的三种同工酶（AAT2a、-2b和-2c）净电荷不同，是由该基因座上两个等位基因AAT2A和AAT2C的表达产生的。利用抗苜蓿AAT2的抗血清，我们之前从一个表达文库中分离出了一个AAT2 cDNA克隆。这个克隆被用作杂交探针，筛选cDNA文库以寻找其他AAT2 cDNA。获得了四个不同的克隆，分别编码AAT2a和AAT2c酶亚基。这两组cDNA编码的多肽净电荷不同，这取决于第296位的氨基酸（缬氨酸或谷氨酸）。在每组等位基因中，两个成员因是否存在一个30 bp（十个氨基酸）序列而彼此不同。这个十个氨基酸序列的有无对成熟AAT2蛋白的大小或电荷没有影响，因为它位于编码该蛋白转运肽的区域内，在转运到质体时会被蛋白水解去除。数据表明，在两个AAT2祖代等位基因中独立发生了一次缺失事件，导致了观察到的四个等位基因cDNA变体。这个十个氨基酸序列的缺失似乎并不损害该酶的正常成熟。
