A-to-I mRNA editing controls disulfide bond formation and the toxicity of the HokB toxin in bacteria

Protein disulfide bonds between cysteine residues serve a prominent role in bacterial protein function, virulence, and viability. Adenosine-to-inosine (A-to-I) mRNA editing changes the genetic information at the RNA level. Previously, we discovered that A-to-I mRNA editing occurs in bacteria (Escherichia coli), identified the mediating enzyme, and showed it may introduce cysteine codons at the mRNA of multiple proteins. However, the ability of A-to-I mRNA editing to recode protein sequence, control disulfide bond formation, and affect its function was never investigated in bacteria. Here, we show that A-to-I mRNA editing constitutes a novel mechanism to control disulfide bond formation that can affect bacterial viability and growth. We focused on the most edited mRNA in E. coli that encodes the self, inner membrane toxin HokB in the HokB/sokB toxin-antitoxin system. First, we showed that A-to-I mRNA editing increases the toxicity of HokB, by recoding a tyrosine to a cysteine at the protein level. Next, we demonstrated that other (DNA-coded) cysteines in HokB and the ability of E. coli to form disulfide bonds are essential to the toxicity of edited HokB. We further showed that edited HokB can induce bacterial death and early entrance to stationary phase. Finally, structural modeling suggests that the mRNA editing-dependent cysteine of one HokB monomer interacts with a DNA-coded cysteine of another monomer, possibly stabilizing the HokB pore. Our work opens new avenues of research in how mRNA editing affects disulfide bond formation in different proteins and their role in bacterial biology.