Department of Microbiology and Molecular Medicine, Faculty of Medicine, University of Geneva, Geneva, Switzerland.
PLoS Genet. 2020 Jul 30;16(7):e1008779. doi: 10.1371/journal.pgen.1008779. eCollection 2020 Jul.
Staphylococcus aureus is an opportunistic pathogen that can grow in a wide array of conditions: on abiotic surfaces, on the skin, in the nose, in planktonic or biofilm forms and can cause many type of infections. Consequently, S. aureus must be able to adapt rapidly to these changing growth conditions, an ability largely driven at the posttranscriptional level. RNA helicases of the DEAD-box family play an important part in this process. In particular, CshA, which is part of the degradosome, is required for the rapid turnover of certain mRNAs and its deletion results in cold-sensitivity. To understand the molecular basis of this phenotype, we conducted a large genetic screen isolating 82 independent suppressors of cold growth. Full genome sequencing revealed the fatty acid synthesis pathway affected in many suppressor strains. Consistent with that result, sublethal doses of triclosan, a FASII inhibitor, can partially restore growth of a cshA mutant in the cold. Overexpression of the genes involved in branched-chain fatty acid synthesis was also able to suppress the cold-sensitivity. Using gas chromatography analysis of fatty acids, we observed an imbalance of straight and branched-chain fatty acids in the cshA mutant, compared to the wild-type. This imbalance is compensated in the suppressor strains. Thus, we reveal for the first time that the cold sensitive growth phenotype of a DEAD-box mutant can be explained, at least partially, by an improper membrane composition. The defect correlates with an accumulation of the pyruvate dehydrogenase complex mRNA, which is inefficiently degraded in absence of CshA. We propose that the resulting accumulation of acetyl-CoA fuels straight-chained fatty acid production at the expense of the branched ones. Strikingly, addition of acetate into the medium mimics the cshA deletion phenotype, resulting in cold sensitivity suppressed by the mutations found in our genetic screen or by sublethal doses of triclosan.
金黄色葡萄球菌是一种机会致病菌，可以在多种条件下生长：在非生物表面、皮肤上、鼻腔中、浮游或生物膜形式下，并可引起多种类型的感染。因此，金黄色葡萄球菌必须能够迅速适应这些不断变化的生长条件，这种能力在很大程度上是由转录后水平驱动的。DEAD-box 家族的 RNA 解旋酶在这个过程中起着重要的作用。特别是，CshA 是降解体的一部分，是某些 mRNA 快速周转所必需的，其缺失导致对冷的敏感性。为了理解这种表型的分子基础，我们进行了大规模的遗传筛选，分离出 82 个独立的冷生长抑制子。全基因组测序揭示了许多抑制子菌株中脂肪酸合成途径受到影响。这一结果与亚致死剂量的三氯生（一种 FASII 抑制剂）部分恢复 cshA 突变体在冷环境中生长的结果一致。参与支链脂肪酸合成的基因的过表达也能抑制冷敏感性。通过气相色谱法分析脂肪酸，我们观察到 cshA 突变体与野生型相比，直链和支链脂肪酸的平衡失调。这种不平衡在抑制子菌株中得到了补偿。因此，我们首次揭示了 DEAD-box 突变体的冷敏感生长表型至少部分可以通过膜组成不当来解释。该缺陷与丙酮酸脱氢酶复合物 mRNA 的积累相关，该复合物在没有 CshA 的情况下不能有效降解。我们提出，由此产生的乙酰辅酶 A 积累促进了直链脂肪酸的产生，而牺牲了支链脂肪酸。引人注目的是，向培养基中添加乙酸可以模拟 cshA 缺失表型，从而抑制我们的遗传筛选中发现的突变或亚致死剂量的三氯生所抑制的冷敏感性。
