Roggiani Manuela, Yadavalli Srujana S, Goulian Mark
Department of Biology, University of Pennsylvania, Philadelphia, PA, United States of America.
PLoS Genet. 2017 Nov 15;13(11):e1007101. doi: 10.1371/journal.pgen.1007101. eCollection 2017 Nov.
Previous studies have shown that exponentially growing Escherichia coli can detect mild acidity (~pH 5.5) and, in response, synthesize enzymes that protect against severe acid shock. This adaptation is controlled by the EvgS/EvgA phosphorelay, a signal transduction system present in virtually every E. coli isolate whose genome has been sequenced. Here we show that, despite this high level of conservation, the EvgS/EvgA system displays a surprising natural variation in pH-sensing capacity, with some strains entirely non-responsive to low pH stimulus. In most cases that we have tested, however, activation of the EvgA regulon still confers acid resistance. From analyzing selected E. coli isolates, we find that the natural variation results from polymorphisms in the sensor kinase EvgS. We further show that this variation affects the pH response of a second kinase, PhoQ, which senses pH differently from the closely related PhoQ in Salmonella enterica. The within-species diversification described here suggests EvgS likely responds to additional input signals that may be correlated with acid stress. In addition, this work highlights the fact that even for highly conserved sensor kinases, the activities identified from a subset of isolates may not necessarily generalize to other members of the same bacterial species.
先前的研究表明，指数生长的大肠杆菌能够检测到轻度酸性环境（约pH 5.5），并相应地合成可抵御严重酸冲击的酶。这种适应性由EvgS/EvgA磷酸化信号转导系统控制，该系统存在于几乎每一株已测序基因组的大肠杆菌分离株中。在此我们表明，尽管该系统具有高度保守性，但EvgS/EvgA系统在pH感应能力方面呈现出惊人的自然变异，一些菌株对低pH刺激完全无反应。然而，在我们测试的大多数情况下，EvgA调控子的激活仍能赋予抗酸性。通过分析选定的大肠杆菌分离株，我们发现这种自然变异源于传感激酶EvgS中的多态性。我们进一步表明，这种变异会影响第二种激酶PhoQ的pH反应，PhoQ感知pH的方式与肠炎沙门氏菌中密切相关的PhoQ不同。这里描述的种内多样性表明，EvgS可能对可能与酸胁迫相关的其他输入信号作出反应。此外，这项工作突出了这样一个事实，即即使对于高度保守的传感激酶，从一部分分离株中确定的活性不一定适用于同一细菌物种的其他成员。
