Ricca E, Limauro D, Lago C T, de Felice M
International Institute of Genetics and Biophysics, Naples, Italy.
J Bacteriol. 1988 Nov;170(11):5197-9. doi: 10.1128/jb.170.11.5197-5199.1988.
The acetohydroxy acid synthase III isozyme, which catalyzes the first common step in the biosynthesis of isoleucine, leucine, and valine in Escherichia coli K-12, is composed of two subunits, the ilvI and ilvH gene products. A missense mutation in ilvH (ilvH612), which reduced the sensitivity of the enzyme to the end product inhibition by valine, also increased its specific activity and lowered the Km for alpha-acetolactate synthesis. The mutation increased the sensitivity of acetohydroxy acid synthase III to dialysis and heat treatment and reduced the requirement for thiamine pyrophosphate addition to the assay mixture for activity. A strain carrying the ilvH612 mutation grew better than a homologous ilvH+ strain in the presence of leucine. The data indicate that this is a consequence of a more active acetohydroxy acid synthase III isozyme rather than the result of an alteration of the leucine-mediated repression of the ilvIH operon.
乙酰羟酸合酶III同工酶催化大肠杆菌K-12中异亮氨酸、亮氨酸和缬氨酸生物合成的首个共同步骤，它由两个亚基组成，即ilvI和ilvH基因产物。ilvH中的一个错义突变（ilvH612）降低了该酶对缬氨酸终产物抑制的敏感性，同时还提高了其比活性并降低了α-乙酰乳酸合成的米氏常数。该突变增加了乙酰羟酸合酶III对透析和热处理的敏感性，并降低了在测定混合物中添加硫胺焦磷酸以激活活性的需求。携带ilvH612突变的菌株在亮氨酸存在的情况下比同源ilvH+菌株生长得更好。数据表明，这是乙酰羟酸合酶III同工酶活性更高的结果，而不是亮氨酸介导的ilvIH操纵子阻遏改变的结果。
