Qi Qin, Preston Gail M, MacLean R Craig
Department of Plant Sciences, University of Oxford, Oxford, United Kingdom.
Department of Zoology, University of Oxford, Oxford, United Kingdom
mBio. 2014 Dec 9;5(6):e01562. doi: 10.1128/mBio.01562-14.
Fitness costs play a key role in the evolutionary dynamics of antibiotic resistance in bacteria by generating selection against resistance in the absence of antibiotics. Although the genetic basis of antibiotic resistance is well understood, the precise molecular mechanisms linking the genetic basis of resistance to its fitness cost remain poorly characterized. Here, we examine how the system-wide impacts of mutations in the RNA polymerase (RNAP) gene rpoB shape the fitness cost of rifampin resistance in Pseudomonas aeruginosa. Rifampin resistance mutations reduce transcriptional efficiency, and this explains 76% of the variation in fitness among rpoB mutants. The pleiotropic consequence of rpoB mutations is that mutants show altered relative transcript levels of essential genes. We find no evidence that global transcriptional responses have an impact on the fitness cost of rifampin resistance as revealed by transcriptome sequencing (RNA-Seq). Global changes in the transcriptional profiles of rpoB mutants compared to the transcriptional profile of the rifampin-sensitive ancestral strain are subtle, demonstrating that the transcriptional regulatory network of P. aeruginosa is robust to the decreased transcriptional efficiency associated with rpoB mutations. On a smaller scale, we find that rifampin resistance mutations increase the expression of RNAP due to decreased termination at an attenuator upstream from rpoB, and we argue that this helps to minimize the cost of rifampin resistance by buffering against reduced RNAP activity. In summary, our study shows that it is possible to dissect the molecular mechanisms underpinning variation in the cost of rifampin resistance and highlights the importance of genome-wide buffering of relative transcript levels in providing robustness against resistance mutations.
Antibiotic resistance mutations carry fitness costs. Relative to the characteristics of their antibiotic-sensitive ancestors, resistant mutants show reduced growth rates and competitive abilities. Fitness cost plays an important role in the evolution of antibiotic resistance in the absence of antibiotics; however, the molecular mechanisms underlying these fitness costs is not well understood. We applied a systems-level approach to dissect the molecular underpinnings of the fitness costs associated with rifampin resistance in P. aeruginosa and showed that most of the variation in fitness cost can be explained by the direct effect of resistance mutations on the enzymatic activity of the mutated gene. Pleiotropic changes in transcriptional profiles are subtle at a genome-wide scale, suggesting that the gene regulatory network of P. aeruginosa is robust in the face of the direct effects of resistance mutations.
适应性代价在细菌抗生素抗性的进化动态中起着关键作用，它在没有抗生素的情况下产生对抗性的选择。虽然抗生素抗性的遗传基础已得到充分理解，但将抗性的遗传基础与其适应性代价联系起来的精确分子机制仍未得到很好的描述。在这里，我们研究了RNA聚合酶（RNAP）基因rpoB中的突变对全系统的影响如何塑造铜绿假单胞菌中利福平抗性的适应性代价。利福平抗性突变降低了转录效率，这解释了rpoB突变体之间适应性差异的76%。rpoB突变的多效性后果是突变体显示出必需基因相对转录水平的改变。我们没有发现证据表明全局转录反应会影响利福平抗性的适应性代价，如转录组测序（RNA-Seq）所揭示的那样。与利福平敏感的祖先菌株的转录谱相比，rpoB突变体转录谱的全局变化很细微，这表明铜绿假单胞菌的转录调控网络对与rpoB突变相关的转录效率降低具有鲁棒性。在较小的尺度上，我们发现利福平抗性突变由于rpoB上游衰减子处终止减少而增加了RNAP的表达，并且我们认为这通过缓冲RNAP活性降低来帮助最小化利福平抗性的代价。总之，我们的研究表明，有可能剖析利福平抗性代价变化背后的分子机制，并强调了全基因组范围内相对转录水平缓冲在提供对抗性突变的鲁棒性方面的重要性。
抗生素抗性突变会带来适应性代价。相对于它们的抗生素敏感祖先的特征，抗性突变体显示出生长速率和竞争能力降低。适应性代价在没有抗生素的情况下抗生素抗性的进化中起着重要作用；然而，这些适应性代价背后的分子机制尚未得到很好的理解。我们应用系统水平的方法来剖析与铜绿假单胞菌中利福平抗性相关的适应性代价的分子基础，并表明适应性代价的大部分变化可以通过抗性突变对突变基因酶活性的直接影响来解释。转录谱的多效性变化在全基因组范围内很细微，这表明铜绿假单胞菌的基因调控网络在面对抗性突变的直接影响时具有鲁棒性。
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