A Synonymous Mutation Upstream of the Gene Encoding a Weak-Link Enzyme Causes an Ultrasensitive Response in Growth Rate
A Synonymous Mutation Upstream of the Gene Encoding a Weak-Link Enzyme Causes an Ultrasensitive Response in Growth Rate
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DOI：
10.1128/jb.00262-16
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发表时间：
2016-10-01
影响因子：
3.2
通讯作者：
Copley, Shelley D.
中科院分区：
文献类型：
--
作者：
Kershner, Jamie P.;McLoughlin, Sean Yu;Copley, Shelley D.
当微生物面临环境挑战或机遇时，可以招募具有混杂次级活性的先前存在的酶来提供新的重要功能。增加新活性效率的突变通常会损害原始活性，导致效率低下的双功能酶。我们研究了当适合度受到这样一种酶E383A ProA(ProA*)限制时，大肠杆菌的生长可以被促进的机制。ProA*可以同时发挥ProA(合成脯氨酸所需的)和ArGC(合成精氨酸所需的)的功能，尽管功能较差。我们确定了四个基因变化，使生长速度提高了6.2倍。Proba操纵子启动子的两个点突变增加了整个操纵子的表达。Proba操纵子周围基因组片段的大量扩增也增加了整个操纵子的表达。最后，ProB编码区的同义点突变为ProA*创造了一个新的启动子。这种同义突变使ProA*的水平增加2倍，但使生长速度增加5倍，这可能是由于两种底物竞争低效的双功能ProA*的活性部位而产生的超敏感反应。重要的是，我们在ProB中发现的高影响同义突变具有显著的两个原因。首先，文献中记载的大多数极地效应都是有害的。这一发现表明，极性效应突变可以产生强烈的有益影响，特别是当一个有机体面临一个艰难的环境挑战时，它对环境的适应能力很差。此外，ProB基因同义突变的结果是ProA水平增加了2倍，但生长速度却增加了5.1倍。虽然在信号网络和遗传回路中经常发现超敏感反应，但对适应性突变的超敏感反应之前还没有报道。
When microbes are faced with an environmental challenge or opportunity, preexisting enzymes with promiscuous secondary activities can be recruited to provide newly important functions. Mutations that increase the efficiency of a new activity often compromise the original activity, resulting in an inefficient bifunctional enzyme. We have investigated the mechanisms by which growth of Escherichia coli can be improved when fitness is limited by such an enzyme, E383A ProA (ProA*). ProA* can serve the functions of both ProA (required for synthesis of proline) and ArgC (required for synthesis of arginine), albeit poorly. We identified four genetic changes that improve the growth rate by up to 6.2-fold. Two point mutations in the promoter of the proBA* operon increase expression of the entire operon. Massive amplification of a genomic segment around the proBA* operon also increases expression of the entire operon. Finally, a synonymous point mutation in the coding region of proB creates a new promoter for proA*. This synonymous mutation increases the level of ProA* by 2-fold but increases the growth rate by 5-fold, an ultrasensitive response likely arising from competition between two substrates for the active site of the inefficient bifunctional ProA*.IMPORTANCEThe high-impact synonymous mutation we discovered in proB is remarkable for two reasons. First, most polar effects documented in the literature are detrimental. This finding demonstrates that polar effect mutations can have strongly beneficial effects, especially when an organism is facing a difficult environmental challenge for which it is poorly adapted. Furthermore, the consequence of the synonymous mutation in proB is a 2-fold increase in the level of ProA* but a disproportionately large 5.1-fold increase in growth rate. While ultrasensitive responses are often found in signaling networks and genetic circuits, an ultrasensitive response to an adaptive mutation has not been previously reported.