Braun M, Henstrand J M, Görlach J, Amrhein N, Schmid J
Institute of Plant Sciences, Swiss Federal Institute of Technology, Zürich, Switzerland.
Planta. 1996;200(1):64-70. doi: 10.1007/BF00196650.
Three plastidic chorismate synthase isozymes (CS1, CS2 and CS2 delta) of tomato were identified by isolation of the corresponding cDNAs. These three cDNAs are derived from only two genes (LeCS1 and LeCS2). This additional complexity results from differential splicing of the primary transcript of one of the genes (LeCS2) giving rise to two different transcripts (CS2 and CS2 delta transcripts). All three isozymes were individually expressed in Escherichia coli both as precursor proteins with N-terminal transit peptides and as mature proteins. Only the mature but not the precursor isozymes CS1 and CS2 were enzymatically active. The enzyme CS2 delta was unstable in E. coli. Both CS1 and CS2 were purified to near homogeneity and their enzymatic properties were analyzed. They differ substantially in their Km values for the substrate 5-enol-pyruvylshikimate 3-phosphate (11 and 80 microM for the mature forms of CS1 and CS2, respectively). The two isozymes appear to be active only as oligomers, and the potential physiological implications of this result are discussed.
通过分离相应的cDNA，鉴定出了番茄的三种质体分支酸合酶同工酶（CS1、CS2和CS2δ）。这三种cDNA仅来自两个基因（LeCS1和LeCS2）。这种额外的复杂性源于其中一个基因（LeCS2）初级转录本的差异剪接，产生了两种不同的转录本（CS2和CS2δ转录本）。所有三种同工酶都分别在大肠杆菌中作为带有N端转运肽的前体蛋白和成熟蛋白进行表达。只有成熟的CS1和CS2同工酶具有酶活性，前体同工酶无活性。酶CS2δ在大肠杆菌中不稳定。CS1和CS2都被纯化至接近均一，并对其酶学性质进行了分析。它们对底物5-烯醇丙酮酸莽草酸-3-磷酸的Km值有很大差异（CS1和CS2成熟形式的Km值分别为11和80μM）。这两种同工酶似乎仅作为寡聚体具有活性，并讨论了该结果潜在的生理学意义。
