RelEB3 toxin–antitoxin system of Salmonella Typhimurium with a ribosome-independent toxin and a mutated non-neutralising antitoxin.

The RelEB3 toxin–antitoxin (TA) system of Salmonella enterica subsp. enterica serovar Typhimurium consists of a RelE3 toxin which suppresses bacterial growth, but its RelB3 antitoxin does not neutralise the toxin. The relEB3 operon is widespread in Proteobacteria and is related to higBA2 from Vibrio cholerae. In contrast to the ribosome-dependent HigB2 toxin, however, the RelE3 toxin degraded free RNA independently of the ribosome. A basic loop possibly involved in HigB2's binding to the ribosome is shortened in RelE3, which instead contains a uniquely conserved R51 important for RelE3's toxicity. The RelB3 antitoxin, meanwhile, specifically recognised the CACC T GGTG palindromic motif in the promoter site. RelB3 contains a unique P14 which is conserved as Ala in most homologues, and mutating P14 to Ala enabled the antitoxin to bind to RelE3 and restored bacterial growth. The P14 RelB3 variant, which most likely arose by a point mutation in a recent ancestor of S. Typhimurium and closely related serovars, could have possibly provided the bacteria with a faster response to stress, and might have spread to other serovars through homologous recombination. • The antitoxin of the RelEB3 toxin–antitoxin system does not neutralise the toxin. • The RelE3 toxin suppressed growth by cleaving RNA independently of the ribosome. • The RelB3 antitoxin recognised the CACC T GGTG palindromic motif. • The P14A mutation restored RelB3's ability to bind RelE3 and neutralise it. • P14 RelB3 might speed up stress response and spread by homologous recombination. [ABSTRACT FROM AUTHOR]