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In this issue of Structure, Kielkopf et al. 1 report the crystal structures of Rhs proteins that are genetically fused to the type VI secretion system PAAR or VgrG proteins. The work highlights the structural similarities of Rhs proteins despite diverse decorations surrounding the Rhs core and different delivery modes., Competing Interests: Declaration of interests The author declares no competing interests., (Copyright © 2024 Elsevier Inc. All rights reserved.)

The E. coli hicAB type II toxin-antitoxin locus is unusual by being controlled by two promoters and by having the toxin encoded upstream of the antitoxin. HicA toxins contain a double-stranded RNA-binding fold and cleaves both mRNA and tmRNA in vivo, while HicB antitoxins contain a partial RNase H fold and either a helix-turn-helix (HTH) or ribbon-helix-helix domain. It is not known how an HTH DNA-binding domain affects higher-order structure for the HicAB modules. Here, we present crystal structures of the isolated E. coli HicB antitoxin and full-length HicAB complex showing that HicB forms a stable DNA-binding module and interacts in a canonical way with HicA despite the presence of an HTH-type DNA-binding domain. No major structural rearrangements take place upon binding of the toxin. Both structures expose well-ordered DNA-binding motifs allowing a model for DNA binding by the antitoxin to be generated., (Copyright © 2019 Elsevier Ltd. All rights reserved.)