Department of Biology, Harvey Mudd College, Claremont, CA 91711, USA.
G3 (Bethesda). 2012 Nov;2(11):1341-4. doi: 10.1534/g3.112.003855. Epub 2012 Nov 1.
The alternative sigma factor RpoS controls a large regulon that allows E. coli to respond to a variety of stresses. Mutations in rpoS can increase rates of nutrient acquisition at the cost of a decrease in stress resistance. These kinds of mutations evolve rapidly under certain laboratory conditions where nutrient acquisition is especially challenging. The frequency of strains lacking RpoS in natural populations of E. coli is less clear. Such strains have been found at frequencies over 20% in some collections of wild isolates. However, laboratory handling can select for RpoS-null strains and may have affected some of these strain collections. Other studies have included an unknown diversity of strains or only used a phenotypic proxy as a measure of RpoS levels. We directly measured RpoS levels in a collection of E. coli that includes the full diversity of the species and that was handled in a manner to minimize the potential for laboratory evolution. We found that only 2% of strains produce no functional RpoS. Comparison of these strains in multiple labs shows that these rpoS mutations occurred in the laboratory. Earlier studies reporting much higher levels of RpoS polymorphism may reflect the storage history of the strains in laboratories rather than true frequency of such strains in natural populations.
替代 sigma 因子 RpoS 控制着一个庞大的调控基因群，使大肠杆菌能够对各种应激做出反应。rpoS 突变可以提高营养物质获取的速度，但会降低对压力的抵抗力。在某些实验室条件下，这些类型的突变会迅速进化，在这些条件下，营养物质的获取尤其具有挑战性。在大肠杆菌的自然种群中，缺乏 RpoS 的菌株的频率不太清楚。在一些野生分离株的集合中，已经发现了超过 20%的此类菌株。然而，实验室处理可能会选择缺乏 RpoS 的菌株，并且可能已经影响了其中一些菌株的集合。其他研究包括了未知多样性的菌株，或者仅使用表型替代物作为 RpoS 水平的衡量标准。我们直接测量了包括该物种所有多样性的大肠杆菌集合中的 RpoS 水平，并且以尽量减少实验室进化的可能性的方式进行了处理。我们发现只有 2%的菌株不能产生功能性的 RpoS。在多个实验室中对这些菌株进行比较表明，这些 rpoS 突变是在实验室中发生的。早期报告 RpoS 多态性水平高得多的研究可能反映了实验室中菌株的储存历史，而不是这种菌株在自然种群中的真实频率。
