School of Biological Sciences, University of Auckland, Auckland, New Zealand.
School of Biological Sciences, University of Canterbury, Christchurch, New Zealand.
Elife. 2023 Apr 6;12:e83845. doi: 10.7554/eLife.83845.
Life requires ribonucleotide reduction for de novo synthesis of deoxyribonucleotides. As ribonucleotide reduction has on occasion been lost in parasites and endosymbionts, which are instead dependent on their host for deoxyribonucleotide synthesis, it should in principle be possible to knock this process out if growth media are supplemented with deoxyribonucleosides. We report the creation of a strain of where all three ribonucleotide reductase operons have been deleted following introduction of a broad spectrum deoxyribonucleoside kinase from Our strain shows slowed but substantial growth in the presence of deoxyribonucleosides. Under limiting deoxyribonucleoside levels, we observe a distinctive filamentous cell morphology, where cells grow but do not appear to divide regularly. Finally, we examined whether our lines can adapt to limited supplies of deoxyribonucleosides, as might occur in the switch from de novo synthesis to dependence on host production during the evolution of parasitism or endosymbiosis. Over the course of an evolution experiment, we observe a 25-fold reduction in the minimum concentration of exogenous deoxyribonucleosides necessary for growth. Genome analysis reveals that several replicate lines carry mutations in and . deoB codes for phosphopentomutase, a key part of the deoxyriboaldolase pathway, which has been hypothesised as an alternative to ribonucleotide reduction for deoxyribonucleotide synthesis. Rather than complementing the loss of ribonucleotide reduction, our experiments reveal that mutations appear that reduce or eliminate the capacity for this pathway to catabolise deoxyribonucleotides, thus preventing their loss via central metabolism. Mutational inactivation of both and is also observed in a number of obligate intracellular bacteria that have lost ribonucleotide reduction. We conclude that our experiments recapitulate key evolutionary steps in the adaptation to life without ribonucleotide reduction.
生命需要核苷酸还原酶来从头合成脱氧核苷酸。由于核苷酸还原酶偶尔会在寄生虫和内共生体中丢失，而这些生物则依赖其宿主合成脱氧核苷酸，因此，如果在生长培养基中添加脱氧核苷，原则上应该可以敲除这个过程。我们报告了一种 的菌株的创建，该菌株在引入来自 的广谱脱氧核苷激酶后，已经删除了所有三个核苷酸还原酶操纵子。我们的菌株在脱氧核苷存在下表现出缓慢但大量的生长。在脱氧核苷有限的情况下，我们观察到一种独特的丝状细胞形态，其中细胞生长但似乎不规律地分裂。最后，我们检查了我们的系是否能够适应脱氧核苷的有限供应，就像在寄生或内共生进化过程中从从头合成到依赖宿主生产的转变中可能发生的那样。在一个进化实验中，我们观察到生长所需的外源性脱氧核苷的最小浓度降低了 25 倍。基因组分析表明，几个重复系在 和 中携带突变。deoB 编码磷酸戊糖异构酶，这是脱氧核糖醛缩酶途径的关键部分，该途径被假设为脱氧核苷酸合成的核苷酸还原酶替代途径。我们的实验表明，突变的出现减少或消除了该途径分解脱氧核苷酸的能力，从而防止它们通过中心代谢途径丢失，而不是补充核苷酸还原酶的缺失。在许多已经丢失核苷酸还原酶的专性细胞内细菌中，也观察到 和 的突变失活。我们得出结论，我们的实验再现了适应没有核苷酸还原酶的生命的关键进化步骤。
