Department of Molecular, Cellular and Developmental Biology, University of California, Santa Barbara, CA 93106-9625, USA.
J Biol Chem. 2012 Aug 24;287(35):29765-75. doi: 10.1074/jbc.M112.374074. Epub 2012 Jul 12.
The translation of non-stop mRNA (which lack in-frame stop codons) represents a significant quality control problem for all organisms. In eubacteria, the transfer-messenger RNA (tmRNA) system facilitates recycling of stalled ribosomes from non-stop mRNA in a process termed trans-translation or ribosome rescue. During rescue, the nascent chain is tagged with the tmRNA-encoded ssrA peptide, which promotes polypeptide degradation after release from the stalled ribosome. Escherichia coli possesses an additional ribosome rescue pathway mediated by the ArfA peptide. The E. coli arfA message contains a hairpin structure that is cleaved by RNase III to produce a non-stop transcript. Therefore, ArfA levels are controlled by tmRNA through ssrA-peptide tagging and proteolysis. Here, we examine whether ArfA homologues from other bacteria are also regulated by RNase III and tmRNA. We searched 431 arfA coding sequences for mRNA secondary structures and found that 82.8% of the transcripts contain predicted hairpins in their 3'-coding regions. The arfA hairpins from Haemophilus influenzae, Proteus mirabilis, Vibrio fischeri, and Pasteurella multocida are all cleaved by RNase III as predicted, whereas the hairpin from Neisseria gonorrhoeae functions as an intrinsic transcription terminator to generate non-stop mRNA. Each ArfA homologue is ssrA-tagged and degraded when expressed in wild-type E. coli cells, but accumulates in mutants lacking tmRNA. Together, these findings show that ArfA synthesis from non-stop mRNA is a conserved mechanism to regulate the alternative ribosome rescue pathway. This strategy ensures that ArfA homologues are only deployed when the tmRNA system is incapacitated or overwhelmed by stalled ribosomes.
不停顿的 mRNA(缺乏有框终止密码子)的翻译对所有生物体都是一个重大的质量控制问题。在真细菌中,转移信使 RNA(tmRNA)系统通过称为转译或核糖体拯救的过程促进从非停顿 mRNA 上停滞的核糖体的循环利用。在拯救过程中,新生链被 tmRNA 编码的 ssrA 肽标记,该肽在从停滞核糖体释放后促进多肽降解。大肠杆菌具有由 ArfA 肽介导的额外的核糖体拯救途径。大肠杆菌 arfA 消息包含一个发夹结构,该结构被 RNase III 切割以产生非停顿转录物。因此,ArfA 水平通过 tmRNA 通过 ssrA-肽标记和蛋白水解来控制。在这里,我们研究了其他细菌的 ArfA 同源物是否也受 RNase III 和 tmRNA 的调节。我们搜索了 431 个 arfA 编码序列的 mRNA 二级结构,发现 82.8%的转录物在其 3'编码区含有预测的发夹。流感嗜血杆菌、奇异变形杆菌、发光杆菌和多杀巴斯德氏菌的 arfA 发夹都如预测的那样被 RNase III 切割,而淋病奈瑟菌的发夹则作为内在转录终止子产生非停顿 mRNA。当在野生型大肠杆菌细胞中表达时,每个 ArfA 同源物都被 ssrA 标记并降解,但在缺乏 tmRNA 的突变体中积累。这些发现表明,从非停顿 mRNA 合成 ArfA 是一种保守的机制,可以调节替代核糖体拯救途径。这种策略可确保仅在 tmRNA 系统失效或被停滞的核糖体淹没时才部署 ArfA 同源物。