The ribosome binding site of a mini‐ ORF protects a T3SS mRNA from degradation by RNase E

Summary Enterohaemorrhagic E scherichia 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 S hine– D algarno element ( SD 2) and a translatable mini‐ ORF of six codons. Disruption of SD 2 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 SD 2 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.