Gong Ming, Cruz-Vera Luis R, Yanofsky Charles
Department of Biological Sciences, Stanford University, Stanford, CA 94305, USA.
J Bacteriol. 2007 Apr;189(8):3147-55. doi: 10.1128/JB.01868-06. Epub 2007 Feb 9.
Upon tryptophan induction of tna operon expression in Escherichia coli, the leader peptidyl-tRNA, TnaC-tRNA(2)(Pro), resists cleavage, resulting in ribosome stalling at the tnaC stop codon. This stalled ribosome blocks Rho factor binding and action, preventing transcription termination in the tna operon's leader region. Plasmid-mediated overexpression of tnaC was previously shown to inhibit cell growth by reducing uncharged tRNA(2)(Pro) availability. Which factors relieve ribosome stalling, facilitate TnaC-tRNA(2)(Pro) cleavage, and relieve growth inhibition were addressed in the current study. In strains containing the chromosomal tna operon and lacking a tnaC plasmid, the overproduction of ribosome recycling factor (RRF) and release factor 3 (RF3) reduced tna operon expression. Their overproduction in vivo also increased the rate of cleavage of TnaC-tRNA(2)(Pro), relieving the growth inhibition associated with plasmid-mediated tnaC overexpression. The overproduction of elongation factor G or initiation factor 3 did not have comparable effects, and tmRNA was incapable of attacking TnaC-tRNA(2)(Pro) in stalled ribosome complexes. The stability of TnaC-tRNA(2)(Pro) was increased appreciably in strains deficient in RRF and RF3 or deficient in peptidyl-tRNA hydrolase. These findings reveal the existence of a natural mechanism whereby an amino acid, tryptophan, binds to ribosomes that have just completed the synthesis of TnaC-tRNA(2)(Pro). Bound tryptophan inhibits RF2-mediated cleavage of TnaC-tRNA(2)(Pro), resulting in the stalling of the ribosome translating tnaC mRNA. This stalling results in increased transcription of the structural genes of the tna operon. RRF and RF3 then bind to this stalled ribosome complex and slowly release TnaC-tRNA(2)(Pro). This release allows ribosome recycling and permits the cleavage of TnaC-tRNA(2)(Pro) by peptidyl-tRNA hydrolase.
在大肠杆菌中,色氨酸诱导tna操纵子表达时,前导肽基 - tRNA,即TnaC - tRNA²(Pro),抵抗切割,导致核糖体在tnaC终止密码子处停滞。这种停滞的核糖体阻止了Rho因子的结合和作用,从而防止了tna操纵子前导区域的转录终止。先前的研究表明,质粒介导的tnaC过表达通过降低无电荷tRNA²(Pro)的可用性来抑制细胞生长。本研究探讨了哪些因素可缓解核糖体停滞、促进TnaC - tRNA²(Pro)切割以及缓解生长抑制。在含有染色体tna操纵子且缺乏tnaC质粒的菌株中,核糖体循环因子(RRF)和释放因子3(RF3)的过量表达降低了tna操纵子的表达。它们在体内的过量表达还提高了TnaC - tRNA²(Pro)的切割速率,缓解了与质粒介导的tnaC过表达相关的生长抑制。延伸因子G或起始因子3的过量表达没有类似的效果,并且tmRNA无法攻击停滞核糖体复合物中的TnaC - tRNA²(Pro)。在缺乏RRF和RF3或缺乏肽基 - tRNA水解酶的菌株中,TnaC - tRNA²(Pro)的稳定性显著增加。这些发现揭示了一种天然机制的存在:氨基酸色氨酸与刚完成TnaC - tRNA²(Pro)合成的核糖体结合。结合的色氨酸抑制RF2介导的TnaC - tRNA²(Pro)切割,导致翻译tnaC mRNA的核糖体停滞。这种停滞导致tna操纵子结构基因的转录增加。然后RRF和RF3结合到这个停滞的核糖体复合物上,并缓慢释放TnaC - tRNA²(Pro)。这种释放允许核糖体循环,并使肽基 - tRNA水解酶能够切割TnaC - tRNA²(Pro)。