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 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 toxin-antitoxin 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 domain. This winged helix-turn-helix is essential for transcriptional repression of the abiE operon. The function of the AbiEi C-terminal domain is poorly characterized, but it contributes to transcriptional repression and is sufficient for toxin neutralization. We demonstrate that a conserved charged surface on one face of the C-terminal domain 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 degrees. These findings demonstrate that the mechanism of DNA binding by the widespread family of AbiEi antitoxins and transcriptional regulators can contribute to negative autoregulation. (C) 2018 Elsevier Ltd. All rights reserved.