Theoretical Computer Science and Algorithmic Methods Group, Institute of Computer Science, University of Göttingen, Goldschmidtstr. 7, 37077, Göttingen, Germany.
Department of Neurobiology, Max-Planck-Institute for Biophysical Chemistry, Am Fassberg 11, 37077, Göttingen, Germany.
BMC Biol. 2021 Dec 4;19(1):258. doi: 10.1186/s12915-021-01197-9.
Yeasts of the CTG-clade lineage, which includes the human-infecting Candida albicans, Candida parapsilosis and Candida tropicalis species, are characterized by an altered genetic code. Instead of translating CUG codons as leucine, as happens in most eukaryotes, these yeasts, whose ancestors are thought to have lost the relevant leucine-tRNA gene, translate CUG codons as serine using a serine-tRNA with a mutated anticodon, [Formula: see text]. Previously reported experiments have suggested that 3-5% of the CTG-clade CUG codons are mistranslated as leucine due to mischarging of the [Formula: see text]. The mistranslation was suggested to result in variable surface proteins explaining fast host adaptation and pathogenicity.
In this study, we reassess this potential mistranslation by high-resolution mass spectrometry-based proteogenomics of multiple CTG-clade yeasts, including various C. albicans strains, isolated from colonized and from infected human body sites, and C. albicans grown in yeast and hyphal forms. Our data do not support a bias towards CUG codon mistranslation as leucine. Instead, our data suggest that (i) CUG codons are mistranslated at a frequency corresponding to the normal extent of ribosomal mistranslation with no preference for specific amino acids, (ii) CUG codons are as unambiguous (or ambiguous) as the related CUU leucine and UCC serine codons, (iii) tRNA anticodon loop variation across the CTG-clade yeasts does not result in any difference of the mistranslation level, and (iv) CUG codon unambiguity is independent of C. albicans' strain pathogenicity or growth form.
Our findings imply that C. albicans does not decode CUG ambiguously. This suggests that the proposed misleucylation of the [Formula: see text] might be as prevalent as every other misacylation or mistranslation event and, if at all, be just one of many reasons causing phenotypic diversity.
CTG 谱系中的酵母，包括感染人类的白色念珠菌、近平滑念珠菌和热带念珠菌，其特征是遗传密码发生了改变。与大多数真核生物不同，这些酵母的祖先被认为已经失去了相关的亮氨酸 tRNA 基因，它们使用带有突变反密码子的丝氨酸 tRNA 将 CUG 密码子翻译成丝氨酸，[公式：见文本]。以前的报告实验表明，由于[公式：见文本]的错误加载，CTG 谱系中的 3-5%的 CUG 密码子被误译为亮氨酸。这种错译被认为导致了表面蛋白的变异，从而解释了快速的宿主适应和致病性。
在这项研究中，我们通过基于高分辨率质谱的蛋白质基因组学方法重新评估了这种潜在的错译，该方法用于多种 CTG 谱系酵母，包括从定植和感染人体部位分离的各种白色念珠菌菌株，以及在酵母和菌丝形式下生长的白色念珠菌。我们的数据不支持 CUG 密码子错译为亮氨酸的偏向性。相反，我们的数据表明：（i）CUG 密码子的错译频率与核糖体错译的正常程度相对应，没有特定氨基酸的偏好；（ii）CUG 密码子与相关的 CUU 亮氨酸和 UCC 丝氨酸密码子一样明确（或不明确）；（iii）CTG 谱系中的酵母 tRNA 反密码子环的变化不会导致错译水平的任何差异；（iv）CUG 密码子的明确性与白色念珠菌菌株的致病性或生长形式无关。
我们的研究结果表明，白色念珠菌不会模糊地解码 CUG。这表明，[公式：见文本]的提议的错亮氨酸化可能像其他任何错氨酰化或错译事件一样普遍，如果有的话，只是导致表型多样性的众多原因之一。
