Beltzer J P, Chang L F, Hinkkanen A E, Kohlhaw G B
J Biol Chem. 1986 Apr 15;261(11):5160-7.
The nucleotide sequence of LEU4, a yeast gene encoding alpha-isopropylmalate synthase, has been determined. An open reading frame of 1857 nucleotides specifies a protein of 619 residues whose calculated molecular weight (68,416) and amino acid composition agree well with earlier estimates based on protein data. The 5' flanking region contains three blocks of sequence potentially involved in the general control of amino acid biosynthesis. It also has six blocks of homology in common with the 5' flanking regions of two other LEU structural genes (LEU1 and LEU2). One of these blocks coincides with a palindromic element that has previously been demonstrated to be important for the specific leucine control of LEU2 (Martinez-Arias, A., Yost, H. J., and Casadaban, M. J. (1984) Nature 307, 740-742). Determination of the 5' ends of the LEU4 transcript indicates the existence of four major and several minor potential transcription start sites. Two of the major sites are located downstream from the ATG at the beginning of the long open reading frame. Utilization of these sites would lead to mRNA that could be translated from an in-frame AUG located 90 nucleotides downstream from the first one. The protein thus generated would be 30 amino acid residues shorter than the larger one. This situation might account for the occurrence of two alpha-isopropylmalate synthase-related proteins observed both in cell-free extracts and in in vitro translation mixtures (Hampsey, D. M., Lewin, A. S., and Kohlhaw, G. B. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 1270-1274). The larger of these proteins was incorporated into the mitochondria while the smaller one was not. We conclude that selection of appropriate transcription and translation start sites might control the subcellular localization of the LEU4 gene product. This conclusion is discussed with respect to other examples in yeast of genes that encode two forms of the same protein.
已确定酵母基因LEU4的核苷酸序列，该基因编码α-异丙基苹果酸合酶。一个1857个核苷酸的开放阅读框指定了一个由619个残基组成的蛋白质，其计算分子量（68,416）和氨基酸组成与早期基于蛋白质数据的估计非常吻合。5'侧翼区域包含三个可能参与氨基酸生物合成一般控制的序列块。它还与其他两个LEU结构基因（LEU1和LEU2）的5'侧翼区域有六个共同的同源性块。其中一个块与一个回文元件重合，该元件先前已被证明对LEU2的特定亮氨酸控制很重要（Martinez-Arias, A., Yost, H. J., and Casadaban, M. J. (1984) Nature 307, 740 - 742）。LEU4转录本5'末端的测定表明存在四个主要和几个次要的潜在转录起始位点。其中两个主要位点位于长开放阅读框起始处的ATG下游。利用这些位点将产生可从第一个位点下游90个核苷酸处的框内AUG翻译的mRNA。由此产生的蛋白质比更大的那个短30个氨基酸残基。这种情况可能解释了在无细胞提取物和体外翻译混合物中观察到的两种α-异丙基苹果酸合酶相关蛋白质的出现（Hampsey, D. M., Lewin, A. S., and Kohlhaw, G. B. (1983) Proc. Natl. Acad. Sci. U.S.A. 80, 1270 - 1274）。这些蛋白质中较大的一种被整合到线粒体中，而较小的一种则没有。我们得出结论，选择合适的转录和翻译起始位点可能控制LEU4基因产物的亚细胞定位。关于酵母中编码同一种蛋白质两种形式的其他基因的例子，讨论了这一结论。
