Laboratory of Genetics, Department of Biosciences and Biotechnology, University of Camerino, 62032 Camerino, Italy.
Laboratory of Genetics, Department of Biosciences and Biotechnology, University of Camerino, 62032 Camerino, Italy.
J Mol Biol. 2016 Apr 24;428(8):1558-73. doi: 10.1016/j.jmb.2016.02.026. Epub 2016 Mar 4.
During the cold adaptation that follows a cold stress, bacterial cells undergo many physiological changes and extensive reprogramming of their gene expression pattern. Bulk gene expression is drastically reduced, while a set of cold shock genes is selectively and transiently expressed. The initial stage of cold acclimation is characterized by the establishment of a stoichiometric imbalance of the translation initiation factors (IFs)/ribosomes ratio that contributes to the preferential translation of cold shock transcripts. Whereas de novo synthesis of the IFs following cold stress has been documented, nothing was known concerning the activity of the rrn operons during the cold acclimation period. In this work, we focus on the expression of the rrn operons and the fate of rRNA after temperature downshift. We demonstrate that in Escherichia coli, rRNA synthesis does not stop during the cold acclimation phase, but continues with greater contribution of the P2 compared to the P1 promoter and all seven rrn operons are active, although their expression levels change with respect to pre-stress conditions. Eight hours after the 37°→10 °C temperature downshift, the newly transcribed rRNA represents up to 20% of total rRNA and is preferentially found in the polysomes. However, with respect to the de novo synthesis of the IFs, both rRNA transcription and maturation are slowed down drastically by cold stress, thereby accounting in part for the stoichiometric imbalance of the IFs/ribosomes. Overall, our data indicate that new ribosomes, which are possibly suitable to function at low temperature, are slowly assembled during cold acclimation.
在冷应激后的冷适应过程中，细菌细胞经历许多生理变化和基因表达模式的广泛重编程。大量基因表达急剧减少，而一组冷休克基因则被选择性地短暂表达。冷适应的初始阶段的特征是建立翻译起始因子 (IFs)/核糖体比例的化学计量失衡，这有助于优先翻译冷休克转录物。虽然已经记录了冷应激后 IFs 的从头合成，但在冷适应期间 rrn 操纵子的活性尚不清楚。在这项工作中，我们专注于 rrn 操纵子的表达以及温度下降后 rRNA 的命运。我们证明，在大肠杆菌中，rRNA 合成在冷适应阶段不会停止，而是继续进行，与 P1 启动子相比，P2 的贡献更大，并且所有七个 rrn 操纵子都活跃，尽管它们的表达水平相对于应激前条件发生了变化。在 37°→10°C 的温度下降 8 小时后，新转录的 rRNA 代表高达 20%的总 rRNA，并且主要存在于多核糖体中。然而，与 IFs 的从头合成相比，冷应激极大地减缓了 rRNA 的转录和成熟，这在一定程度上导致了 IFs/核糖体的化学计量失衡。总的来说，我们的数据表明，新的核糖体可能适合在低温下发挥作用，在冷适应期间会缓慢组装。
