Wek R C, Hauser C A, Hatfield G W
Nucleic Acids Res. 1985 Jun 11;13(11):3995-4010. doi: 10.1093/nar/13.11.3995.
Three acetohydroxy acid synthase isozymes, AHAS I (ilvBN), AHAS II (ilvGM) and AHAS III (ilvIH) catalyze the first step of the parallel isoleucine-valine biosynthetic pathway in Escherichia coli. Previous DNA sequence and protein purification data have shown that AHAS II and AHAS III are composed of large and small subunits encoded in the ilvGMEDA and ilvIH operons, respectively. Recent protein purification and characterization data have demonstrated that the AHAS I isozyme is also composed of large and small subunits (L. Eoyang, L. and P. M. Silverman [1984] J. Bacteriol. 157:184-189). Now the complete DNA sequence of the operon encoding the AHAS I isozyme has been determined. These data show that both AHAS I subunits (Mr 60,400 and Mr 11,100) are encoded in this operon. The coordinant regulation of both genes of the ilvBN operon has also been demonstrated. Comparisons of the DNA sequences of the genes encoding all three AHAS isozymes have been performed. Conserved homologies were observed between both the large and small subunits of all three isozymes. The closest homology was seen between the AHAS I and AHAS II isozymes. On the basis of these comparisons a rationale for the evolution of the AHAS isozymes in E. coli has been proposed.
三种乙酰羟酸合酶同工酶，AHAS I（ilvBN）、AHAS II（ilvGM）和AHAS III（ilvIH）催化大肠杆菌中异亮氨酸 - 缬氨酸平行生物合成途径的第一步。先前的DNA序列和蛋白质纯化数据表明，AHAS II和AHAS III分别由ilvGMEDA和ilvIH操纵子中编码的大亚基和小亚基组成。最近的蛋白质纯化和表征数据表明，AHAS I同工酶也由大亚基和小亚基组成（L. Eoyang，L.和P. M. Silverman [1984] J. Bacteriol. 157:184 - 189）。现在已经确定了编码AHAS I同工酶的操纵子的完整DNA序列。这些数据表明，AHAS I的两个亚基（60,400 Mr和11,100 Mr）都在这个操纵子中编码。ilvBN操纵子的两个基因的协同调节也得到了证实。已经对编码所有三种AHAS同工酶的基因的DNA序列进行了比较。在所有三种同工酶的大亚基和小亚基之间都观察到了保守的同源性。AHAS I和AHAS II同工酶之间的同源性最为接近。基于这些比较，提出了大肠杆菌中AHAS同工酶进化的理论依据。
