During a screen for respiration competent yeast mutants that were unable to grow with acetate as a carbon source, two idh2 cit1 double mutants were identified. These strains were defective in the catalytic subunit of the NAD(+)-dependent isocitrate dehydrogenase and citrate synthase of the tricarboxylic acid (TCA) cycle. The strains harboring the idh2 alleles from these strains had two unusual phenotypes. First, their growth on many nonfermentable carbon sources was much poorer than strains containing other idh2 mutations. Second, the poor growth phenotype could be suppressed by the presence of mutations in CIT1 and other genes encoding oxidative functions. Spontaneous suppressor mutants that restore fast growth on glycerol medium to strains harboring two idh2 alleles were isolated, and a large percentage of the suppressor mutations have been identified within the CIT1 gene and at several other loci. Elevated levels of several TCA cycle proteins were observed in these idh2 mutants that were not observed in the presence of suppressing cit1 mutations. Citrate and isocitrate concentrations were also elevated in the idh2 mutants, but probably not to toxic levels. Five idh2 alleles were sequenced to understand the defects of the two classes of mutations. Sequence analysis indicated that the poor growth phenotype was caused by the loss of Idh2p protein. Similarly, eight cit1 alleles were sequenced to understand their characteristics as glycerol suppressors of idh2. These and other studies indicate that any mutation within CIT1 was capable of suppressing the idh2 mutations. Several models to explain these interactions are discussed.

译文

在对呼吸的筛选中,不能用乙酸盐作为碳源生长的能干酵母突变体,鉴定出两个idh2 cit1双突变体。这些菌株在三羧酸(TCA)循环的NAD()依赖性异柠檬酸脱氢酶和柠檬酸合酶的催化亚基中存在缺陷。包含来自这些菌株的idh2等位基因的菌株具有两种不同的表型。首先,它们在许多不可发酵碳源上的生长要比含有其他idh2突变的菌株差得多。第二,生长不良的表型可以被CIT1和其他编码氧化功能的基因突变的存在所抑制。分离出自发性抑制突变体,该突变体可在甘油培养基上恢复到含有两个idh2等位基因的菌株的快速生长,并且已在CIT1基因和其他几个基因座中鉴定出很大比例的抑制突变。在这些idh2突变体中观察到几种TCA循环蛋白的水平升高,而在抑制cit1突变的情况下未观察到。在idh2突变体中柠檬酸和异柠檬酸浓度也升高,但可能没有达到毒性水平。对五个idh2等位基因进行了测序,以了解这两类突变的缺陷。序列分析表明,不良的生长表型是由Idh2p蛋白的丢失引起的。同样,对八个cit1等位基因进行了测序,以了解其作为idh2甘油抑制剂的特征。这些研究和其他研究表明,CIT1中的任何突变都能够抑制idh2突变。讨论了解释这些相互作用的几种模型。

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