Davis R H, Ristow J L, Ginsburgh C L
Mol Gen Genet. 1981;181(2):215-21. doi: 10.1007/BF00268429.
Carbamyl phosphate synthetase A is a two-polypeptide, mitochondrial enzyme of arginine synthesis in Neurospora. The large subunit is encoded in the arg-3 locus and can catalyze formation of carbamyl-P with ammonia as the N donor. The small subunit is encoded in the unlinked arg-2 locus and imparts to the holoenzyme the ability to use glutamine, the biological substrate, as the N donor. By using nonsense mutations of arg-3, it was shown that the small subunit of the enzyme enters the mitochrondrion independently and is regulated in the same manner as it is in wild type. Similarly, arg-2 mutations, affecting the small subunit, have no effect on the localization or the regulation of the large subunit. The two subunits are regulated differently. Like most polypeptides of the pathway, the large subunit is not repressible and derepresses 3- to 5-fold upon arginine-starvation of mycelia. In contrast, the glutamine-dependent activity of the holoenzyme is fully repressible and has a range of variation of over 100-fold. In keeping with this behavior, it is shown here that the small polypeptide, as visualized on two-dimensional gels, is also fully repressible. We conclude that the two subunits of the enzyme are localized independently, controlled independently and over different ranges, and that aggregation kinetics cannot alone explain the unusual regulatory amplitude of the native, two-subunit enzyme. The small subunit molecular weight was shown to be approximately 45,000.
氨甲酰磷酸合成酶A是一种由两条多肽组成的线粒体酶，参与粗糙脉孢菌中精氨酸的合成。大亚基由arg-3基因座编码，能以氨作为氮供体催化氨甲酰磷酸的形成。小亚基由不连锁的arg-2基因座编码，赋予全酶使用谷氨酰胺（生物底物）作为氮供体的能力。通过使用arg-3的无义突变，研究表明该酶的小亚基独立进入线粒体，并且其调控方式与野生型相同。同样，影响小亚基的arg-2突变对大亚基的定位或调控没有影响。这两个亚基的调控方式不同。与该途径的大多数多肽一样，大亚基不可被阻遏，在菌丝体精氨酸饥饿时会去阻遏3至5倍。相反，全酶的谷氨酰胺依赖性活性可被完全阻遏，变化范围超过100倍。与此行为一致，本文表明二维凝胶上显示的小多肽也可被完全阻遏。我们得出结论，该酶的两个亚基独立定位、独立控制且调控范围不同，并且聚集动力学不能单独解释天然双亚基酶异常的调控幅度。已证明小亚基的分子量约为45,000。
