Comer M M, Dondon J, Graffe M, Yarchuk O, Springer M
Institut de Biologie Physico-Chimique, Paris, France.
J Mol Biol. 1996 Aug 16;261(2):108-24. doi: 10.1006/jmbi.1996.0445.
The expression of the gene thrS encoding threonyl-tRNA synthetase is under the control of two apparently different regulatory loops: translational feedback regulation and growth rate-dependent control. The translational feedback regulation is due to the binding of threonyl-tRNA synthetase to a site located in the leader RNA of thrS, upstream of the initiation codon, which mimics the anticodon stem and loop of tRNA(Thr). This binding competes with that of the ribosome and thus inhibits translation initiation. Here, we investigate the mechanism of growth rate-dependent control, i.e. the mechanism by which the synthetase accumulates at high growth rates. We show that growth rate-dependent control acts at the level of translation and requires feedback regulation since mutations that abolish feedback regulation also abolish growth rate-dependent control. We also show that tRNA(Thr), which accumulates at high growth rates, is one of the effectors of growth rate-dependent control since its accumulation can cause derepression independently of growth rate. We show that this tRNA(Thr)-dependent derepression is also dependent on feedback regulation since mutations which abolish feedback also prevent derepression. Based on these results and previous data concerning the mechanism of translational feedback regulation, we propose that threonyl-tRNA synthetase growth rate-dependent control is the consequence of the accumulation at high growth rates of two effectors, the ribosome and tRNA(Thr). We also study the growth rate-dependence of the steady state level of thrS mRNA and show that the steady state level of thrS mRNA increases at high growth rates. This increase is dependent on the translational feedback regulation and can also be detected, independently of growth rate, when thrS mRNA translation is derepressed. Consistently with the model of growth rate-dependent control above, we propose that at high growth rates, the mRNA is well translated and thus stabilised and that, at low growth rates, because of its low translation, thrS mRNA is rapidly degraded.
编码苏氨酰 - tRNA合成酶的基因thrS的表达受两个明显不同的调控环控制：翻译反馈调控和生长速率依赖性调控。翻译反馈调控是由于苏氨酰 - tRNA合成酶与位于thrS前导RNA中起始密码子上游的一个位点结合，该位点模拟了tRNA（Thr）的反密码子茎环结构。这种结合与核糖体的结合相互竞争，从而抑制翻译起始。在此，我们研究生长速率依赖性调控的机制，即合成酶在高生长速率下积累的机制。我们表明，生长速率依赖性调控在翻译水平起作用，并且需要反馈调控，因为消除反馈调控的突变也会消除生长速率依赖性调控。我们还表明，在高生长速率下积累的tRNA（Thr）是生长速率依赖性调控的效应物之一，因为它的积累可以独立于生长速率导致去阻遏。我们表明，这种依赖于tRNA（Thr）的去阻遏也依赖于反馈调控，因为消除反馈的突变也会阻止去阻遏。基于这些结果以及先前关于翻译反馈调控机制的数据，我们提出苏氨酰 - tRNA合成酶生长速率依赖性调控是两种效应物（核糖体和tRNA（Thr））在高生长速率下积累的结果。我们还研究了thrS mRNA稳态水平的生长速率依赖性，并表明thrS mRNA的稳态水平在高生长速率下增加。这种增加依赖于翻译反馈调控，并且当thrS mRNA翻译去阻遏时，也可以独立于生长速率检测到。与上述生长速率依赖性调控模型一致，我们提出在高生长速率下，mRNA被有效翻译并因此稳定，而在低生长速率下，由于其低翻译水平，thrS mRNA迅速降解。
