Txe, an endoribonuclease of the enterococcal Axe–Txe toxin–antitoxin system, cleaves mRNA and inhibits protein synthesis

Abstract

Theaxe–txeoperon encodes a toxin–antitoxin (TA) pair, Axe–Txe, that was initially identified on the multidrug-resistance plasmid pRUM inEnterococcus faecium. InEscherichia coli, expression of the Txe toxin is known to inhibit cell growth, and co-expression of the antitoxin, Axe, counteracts the toxic effect of Txe. Here, we report the nucleotide sequence of pS177, a 39 kb multidrug-resistant plasmid isolated from vancomycin-resistantEnt. faecium, which harbours theaxe–txeoperon and thevanAgene cluster. RT-PCR analysis revealed that theaxe–txetranscript is produced by strain S177 as well as by other vancomycin-resistant enteroccoci. Moreover, we determine the mechanism by which the Txe protein exerts its toxic activity. Txe inhibits protein synthesis inE. coliwithout affecting DNA or RNA synthesis, and inhibits protein synthesis in a cell-free system. Usingin vivoprimer extension analysis, we demonstrate that Txe preferentially cleaves single-stranded mRNA at the first base after an AUG start codon. We conclude that Txe is an endoribonuclease which cleaves mRNA and inhibits protein synthesis.