Previously, we identified 49 undeletable chromosomal regions harboring only non-essential genes in the genome of Saccharomyces cerevisiae. We proposed that there might be unknown synthetic lethal combinations of genes present in such undeletable regions of the genome. In this study, we chose four of the smallest undeletable chromosomal regions among the 49 and performed extensive further analyses to narrow down the gene-pairs responsible for lethality by replacing sub-regions in various combinations with a DNA module comprising the CgLEU2 marker. Although the methodology was different from previous study, interestingly the results revealed that not only the sub-regions but also the entire region was replaceable. To solve the apparent discrepancy between previous and present results, we further conducted additional analysis including investigation of suppressor mutation and mini-chromosome loss assay through the construction of mini-chromosome harboring two particular chromosomal regions with marked with URA3 marker by employing 5-FOA system. Based upon careful observation on the phenotype of colony formation on 5-FOA medium by spot test, we came to an important conclusion that particular chromosomal regions harboring only non-essential genes can be categorized into three classes, i.e., essential, non-essential and intrinsically essential. Intrinsically essential region is defined as appearance of papillae after mini-chromosome loss which implicates that the region is essential but compensatable against cell lethality. Our present study indicates that prudent and multiple approaches as performed in this study are needed to judge whether a particular chromosomal region of the S. cerevisiae genome is essential, non-essential or intrinsically essential but compensatable.

中文翻译：

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评估酿酒酵母中是否存在不可删除的染色体区域以实现细胞活力的系统方法。

以前，我们确定了49个不可删除的染色体区域，这些区域仅包含酿酒酵母基因组中的非必需基因。我们提出在基因组的此类不可删除区域中可能存在未知的合成致死基因组合。在这项研究中，我们选择了49个中最小的不可删除染色体区域中的四个，并进行了广泛的进一步分析，通过用包含CgLEU2标记的DNA组件替换各种组合中的子区域，缩小了致死的基因对。尽管方法与以前的研究不同，但有趣的是，结果表明不仅子区域而且整个区域都是可替换的。为了解决以前和现在的结果之间明显的差异，我们进一步进行了进一步的分析，包括通过使用5-FOA系统构建包含两个带有URA3标记的特定染色体区域的微型染色体，构建抑制子突变和微型染色体损失检测方法。通过点检仔细观察5-FOA培养基上菌落形成的表型，我们得出一个重要的结论，即仅包含非必需基因的特定染色体区域可分为三类，即必需，非必需和非必需。本质上至关重要。本质上本质上的区域被定义为微型染色体丢失后的乳头状外观，这暗示该区域是必需的，但可补偿细胞致死性。