Department of Molecular Microbiology and Biotechnology, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel.
PLoS Genet. 2018 Mar 12;14(3):e1007283. doi: 10.1371/journal.pgen.1007283. eCollection 2018 Mar.
Listeria monocytogenes (Lm) is a saprophyte and intracellular pathogen. Transition to the pathogenic state relies on sensing of host-derived metabolites, yet it remains unclear how these are recognized and how they mediate virulence gene regulation. We previously found that low availability of isoleucine signals Lm to activate the virulent state. This response is dependent on CodY, a global regulator and isoleucine sensor. Isoleucine-bound CodY represses metabolic pathways including branched-chain amino acids (BCAA) biosynthesis, however under BCAA depletion, as occurs during infection, BCAA biosynthesis is upregulated and isoleucine-unbound CodY activates virulence genes. While isoleucine was revealed as an important input signal, it was not identified how internal levels are controlled during infection. Here we show that Lm regulates BCAA biosynthesis via CodY and via a riboregulator located upstream to the BCAA biosynthesis genes, named Rli60. rli60 is transcribed when BCAA levels drop, forming a ribosome-mediated attenuator that cis-regulates the downstream genes according to BCAA supply. Notably, we found that Rli60 restricts BCAA production, essentially starving Lm, a mechanism that is directly linked to virulence, as it controls the internal isoleucine pool and thereby CodY activity. This controlled BCAA auxotrophy likely evolved to enable isoleucine to serve as a host signal and virulence effector.
李斯特菌（Lm）是一种腐生菌和细胞内病原体。向致病性状态的转变依赖于对宿主来源代谢物的感应，但这些如何被识别以及它们如何介导毒力基因调控仍不清楚。我们之前发现，异亮氨酸的低可用性信号提示李斯特菌激活毒力状态。这种反应依赖于 CodY，这是一种全局调节剂和异亮氨酸传感器。异亮氨酸结合的 CodY 抑制包括支链氨基酸（BCAA）生物合成在内的代谢途径，但在 BCAA 耗尽时，如感染期间，BCAA 生物合成上调，异亮氨酸非结合的 CodY 激活毒力基因。虽然异亮氨酸被揭示为一个重要的输入信号，但在感染过程中，内部水平如何得到控制仍未确定。在这里，我们表明李斯特菌通过 CodY 和位于 BCAA 生物合成基因上游的核糖调节因子 Rli60 来调节 BCAA 生物合成。当 BCAA 水平下降时，rli60 转录，形成一个核糖体介导的衰减子，根据 BCAA 的供应，对下游基因进行顺式调节。值得注意的是，我们发现 Rli60 限制了 BCAA 的产生，基本上使李斯特菌饥饿，这一机制与毒力直接相关，因为它控制内部异亮氨酸池，从而控制 CodY 活性。这种受控的 BCAA 营养缺陷可能是为了使异亮氨酸能够作为宿主信号和毒力效应物而进化而来的。
