Berg C M, Wang M D, Vartak N B, Liu L
Department of Molecular and Cell Biology, University of Connecticut, Storrs 06268.
Gene. 1988 May 30;65(2):195-202. doi: 10.1016/0378-1119(88)90456-8.
The four general transaminases of Escherichia coli K-12 have overlapping, but discrete, substrate specificities and participate in the final step in the synthesis of at least seven different amino acids. Through the use of strains that have mutations in one or more transaminase genes and carry a different wild-type (wt) gene on a multicopy plasmid, it was possible to detect instances in which an amplified wt gene suppressed nonallelic mutations. In these cases, overproduction of the enzyme permitted a broader range of substrates to be used at physiologically significant levels, either because a low catalytic efficiency (in the case analyzed here) or a low affinity of the enzyme towards the substrate prevented its effective utilization under normal conditions. Consequently, by compensating for a low catalytic reaction rate, enzyme overproduction circumvents the original lesion and restores biosynthetic activity to the mutant strain. The suppression of a mutation in one gene by amplified copies of a different wt gene is termed 'multicopy suppression'. This phenomenon is useful for detecting poorly expressed genes, for detecting duplicate genes, for identifying secondary functions of the products of known genes, and for elucidating the metabolic role of the product of the suppressed gene.
大肠杆菌K-12的四种转氨酶具有重叠但又有区别的底物特异性，并且参与至少七种不同氨基酸合成的最后一步。通过使用在一个或多个转氨酶基因中发生突变并在多拷贝质粒上携带不同野生型（wt）基因的菌株，有可能检测到扩增的wt基因抑制非等位基因突变的情况。在这些情况下，酶的过量产生使得在生理相关水平上能够使用更广泛的底物，这要么是因为催化效率低（在此处分析的情况中），要么是因为酶对底物的亲和力低，从而在正常条件下阻止了其有效利用。因此，通过补偿低催化反应速率，酶的过量产生规避了原始损伤并恢复了突变菌株的生物合成活性。一个不同的wt基因的扩增拷贝对一个基因中的突变的抑制被称为“多拷贝抑制”。这种现象对于检测低表达基因、检测重复基因、鉴定已知基因产物的次要功能以及阐明被抑制基因产物的代谢作用很有用。
