AbiEi binds cooperatively to the Type IV abiE toxin-antitoxin operator via a positively-charged surface and causes DNA bending and negative autoregulation

Abstract

Bacteria resist phage infection using multiple strategies, including CRISPR-Cas and abortive infection (Abi) systems. Abi systems provide population-level protection from phage predation, via “altruistic” cell suicide. It has recently been shown that some Abi systems function via a toxin-antitoxin (TA) mechanism, such as the widespread AbiE family. The Streptococcus agalactiae AbiE system consists of a bicistronic operon encoding the AbiEi antitoxin and AbiEii toxin, which function as a Type IV TA system. Here we examine the AbiEi antitoxin, which belongs to a large family of transcriptional regulators with a conserved N-terminal winged-helix-turn-helix (wHTH) domain. This wHTH is essential for transcriptional repression of the abiE operon. The function of the AbiEi C-terminal domain (CTD) is poorly characterised, but it contributes to transcriptional repression and is sufficient for toxin neutralization. We demonstrate that a conserved charged surface on one face of the CTD assists sequence-specific DNA binding and negative autoregulation, without influencing antitoxicity. Furthermore, AbiEi binds cooperatively to two inverted repeats within the abiE promoter and bends the DNA by 72°. These findings demonstrate the mechanism of DNA binding by the widespread family of AbiEi antitoxins and transcriptional regulators can contribute to negative autoregulation.