Growth Rate-dependent Control, Feedback Regulation and Steady-state mRNA Levels of the Threonyl-tRNA Synthetase Gene of Escherichia coli

The expression of the gene thrSencoding 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 de- repression 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 thrSmRNA and show that the steady state level of thrSmRNA increases at high growth rates. This increase is dependent on the translational feedback regulation and can also be detected, independently of growth rate, when thrSmRNA 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, thrSmRNA is rapidly degraded.