When we can’t trust bacterial DNA:
The role of mRNA editing in bacteria
2Department of Molecular Genetics, Weizmann Institute of Science, Israel
Adenosine-to-inosine (A-to-I) mRNA editing is a natural process that can recode protein sequences because the ribosome identifies inosine (I) as guanosine (G). Until recently, however, A-to-I mRNA editing was considered absent in prokaryotes.
Recently, we showed that mRNA editing is not confined to eukaryotes but occurs in bacterial (Escherichia coli) mRNAs. All identified sites in protein-coding regions change tyrosine to a cysteine codon, suggesting that it can recode protein sequences. Focusing on one of the edited transcripts, which encodes a self-killing toxin (HokB) in a toxin-antitoxin system, revealed that mRNA editing enhances its toxicity. Therefore, having a cysteine rather than a tyrosine at the edited site in HokB affects its function. However, the molecular mechanism behind the edited-dependent toxicity of HokB is still unknown.
Here, we show that edited HokB likely forms a disulfide bond, leading to its hyper toxicity. We mutated a second site in HokB (C46S), which abolished the toxicity of edited HokB. Similarly, edited HokB was not toxic in a strain that cannot form disulfide bonds (ΔdsbA). Supplementing DsbA from a plasmid in the ΔdsbA strain restored the observed toxicity suggesting that the function of DsbA is crucial for the observed toxicity of edited HokB. Previously, we identified more than 150 possible mRNA editing events that recode tyrosine to cysteine codons. Therefore, we suggest that mRNA editing may regulate disulfide bond formation in other bacterial proteins and affect their function.