Nonsense Mutations in cspA Cause Ribosome Trapping Leading to Complete Growth Inhibition and Cell Death at Low Temperature in Escherichia coli

Abstract

CspA, the major cold shock protein of Escherichia coli, is dramatically induced immediately after cold shock. CspA production is transient and reduces to a low basal level when cells become adapted. Here we show that expression from multicopy plasmids of mutant cspA mRNAs bearing nonsense mutations in the coding region caused sustained high levels of the mutant mRNAs at low temperature, resulting in complete inhibition of cell growth ultimately leading to cell death. We demonstrate that the observed growth inhibition was caused by largely exclusive occupation of cellular ribosomes by the mutant cspA mRNAs. Such sequestration of ribosomes even occurs without a single peptide bond formation, implying that the robust translatability of the cspA mRNA is determined at the step of initiation. Further analysis demonstrated that the downstream box of the cspA mRNA was dispensable for the effect, whereas the upstream box of the mRNA was essential. Our system may offer a novel means to study sequence or structural elements involved in the translation of the cspA mRNA and may also be utilized to regulate bacterial growth at low temperature.