Department of Microbiology and Immunology, University of Maryland School of Medicine, Baltimore, MD 21201, USA.
Mol Microbiol. 2012 Dec;86(5):1167-82. doi: 10.1111/mmi.12050. Epub 2012 Oct 12.
Enterohaemorrhagic Escherichia coli harbours a pathogenicity island encoding a type 3 secretion system used to translocate effector proteins into the cytosol of intestinal epithelial cells and subvert their function. The structural proteins of the translocon are encoded in a major espADB mRNA processed from a precursor. The translocon mRNA should be highly susceptible to RNase E cleavage because of its AU-rich leader region and monophosphorylated 5'-terminus, yet it manages to avoid rapid degradation. Here, we report that the espADB leader region contains a strong Shine-Dalgarno element (SD2) and a translatable mini-ORF of six codons. Disruption of SD2 so as to weaken ribosome binding significantly reduces the concentration and stability of esp mRNA, whereas codon substitutions that impair translation of the mini-ORF have no such effect. These findings suggest that occupancy of SD2 by ribosomes, but not mini-ORF translation, helps to protect espADB mRNA from degradation, likely by hindering RNase E access to the AU-rich leader region.
产肠出血性大肠杆菌带有一个编码 III 型分泌系统的致病岛，该系统用于将效应蛋白易位到肠上皮细胞的细胞质中，并颠覆其功能。转位器的结构蛋白由从前体加工而来的主要 espADB mRNA 编码。由于其富含 AU 的前导区和单磷酸化的 5'-末端，转位器 mRNA 应该非常容易被 RNase E 切割，但它设法避免了快速降解。在这里，我们报告 espADB 前导区包含一个强 Shine-Dalgarno 元件 (SD2) 和一个可翻译的六个密码子的迷你 ORF。破坏 SD2 以显著削弱核糖体结合会大大降低 esp mRNA 的浓度和稳定性，而破坏迷你 ORF 翻译的密码子替换则没有这种作用。这些发现表明，核糖体占据 SD2，但不是迷你 ORF 翻译，有助于保护 espADB mRNA 免受降解，可能是通过阻碍 RNase E 进入富含 AU 的前导区。
