Case M E, Giles N H
Genetics. 1974 Aug;77(4):613-26. doi: 10.1093/genetics/77.4.613.
Extensive genetical and biochemical studies have been performed with revertants and secondary arom-2 mutants induced in two different primary non-complementing mutants which map within the arom gene cluster of Neurospora crassa. These studies indicate that mutant M54 but not M25 can revert by super-suppressor mutations in unlinked genes, thus confirming previous evidence that M54 contains a nonsense codon. At least three new super suppressors of M54 have been detected. All four super suppressors (including one previously detected) when combined with M54 result in high levels of all five of the arom enzymic activities in the form of arom multienzyme complexes very similar to (but not necessarily identical with) that in wild type (WT).-Evidence has also been obtained that the two non-complementing mutants can yield revertants which appear to result from true back mutations and produce arom aggregates essentially indistinguishable from that of WT. In addition, M25, but not M54, when plated on quinic acid yields revertants (secondary mutants) some of which are phenotypically indistinguishable from arom-2 primary mutants and others of which, although also mapping within the arom-2 gene, exhibit unusual properties. Genetic evidence indicates that the M25 secondary mutants are localized within the arom-2 gene, but that they arise from mutational events more complex than ones resulting in single base pair changes in the M25 codon.-The recovery of secondary arom-2 mutants as revertants of non-complementing arom mutants provides strong evidence, independent of earlier recombination data, that non-complementing arom mutants are located within the arom-2 structural gene of the arom gene cluster. In addition, the occurrence and characteristics of these secondary arom-2 mutants provide strong evidence, independent of the results with nonsense suppressors, that the arom gene cluster is transcribed, beginning with the arom-2 gene, as a single polycistronic messenger ribonucleic acid (mRNA) molecule which is subsequently translated into the arom multienzyme complex.
对在粗糙脉孢菌芳香基因簇内定位的两个不同的初级非互补突变体中诱导产生的回复突变体和次级芳香-2突变体进行了广泛的遗传学和生化研究。这些研究表明，突变体M54而非M25可通过非连锁基因中的超抑制突变回复，从而证实了先前的证据，即M54含有一个无义密码子。已检测到至少三种M54的新超抑制子。所有四种超抑制子（包括先前检测到的一种）与M54结合时，会以非常类似于（但不一定与）野生型（WT）的芳香多酶复合物形式产生高水平的所有五种芳香酶活性。还获得了证据表明，这两个非互补突变体可产生回复突变体，这些回复突变体似乎是由真正的回复突变产生的，并且产生的芳香聚集体与WT的基本无法区分。此外，当M25接种在奎尼酸上时会产生回复突变体（次级突变体），其中一些在表型上与芳香-2初级突变体无法区分，而其他一些虽然也定位在芳香-2基因内，但表现出不同寻常的特性。遗传证据表明，M25次级突变体位于芳香-2基因内，但它们是由比导致M25密码子中单碱基对变化的突变事件更复杂突变事件产生的。作为非互补芳香突变体的回复突变体回收次级芳香-2突变体，提供了独立于早期重组数据的有力证据，即非互补芳香突变体位于芳香基因簇的芳香-2结构基因内。此外，这些次级芳香-2突变体的出现和特征提供了独立于无义抑制子结果的有力证据，即芳香基因簇从芳香-2基因开始转录为单个多顺反子信使核糖核酸（mRNA）分子，随后被翻译成芳香多酶复合物。
