Cleavage of mRNAs and role of tmRNA system under amino acid starvation in Escherichia coli.

We have shown previously that ribosome stalling during translation caused by various reasons leads to mRNA cleavage, resulting in non-stop mRNAs that are eliminated in a tmRNA-dependent manner. Amino acid starvation is a physiological condition in which ribosome stalling is expected to occur more frequently. Here we demonstrate that mRNA cleavage is induced by amino acid starvation, resulting in accumulation of truncated mRNAs in cells lacking tmRNA. The truncated mRNAs are eliminated in wild-type cells, indicating that the tmRNA system rapidly degrade the truncated mRNAs. The cleavage pattern of model mRNAs in which serine codons were replaced with threonine codons indicated that mRNA cleavage occurs near serine codons in response to serine starvation. Cells lacking all of the five known toxin loci were proficient in mRNA cleavage, showing that toxin-antitoxin systems are not responsible for the cleavage. A mild serine starvation caused a significant growth inhibition in cells lacking tmRNA but not in wild-type cells. The ribosome-mediated mRNA cleavage along with the tmRNA system is an important mechanism that enables cells to adapt to amino acid starvation conditions.