Convergent Evolution of the Barnase/EndoU/Colicin/RelE (BECR) Fold in Antibacterial tRNase Toxins

• Published in: Structure (London) 2019-11, Vol.27 (11)
• Main Authors: Gucinski, Grant C., Michalska, Karolina, Garza-Sánchez, Fernando, Eschenfeldt, William H., Stols, Lucy, Nguyen, Josephine Y., Goulding, Celia W., Joachimiak, Andrzej, Hayes, Christopher S.
• Format: Article
• Language: English
• Subjects: bacterial competition; BASIC BIOLOGICAL SCIENCES; toxin-antitoxin systems; two-partner secretion; type V secretion system
• ISSN: 0969-2126; 1878-4186

Contact-dependent growth inhibition (CDI) is a form of interbacterial competition mediated by CdiB-CdiA two-partner secretion systems. CdiA effector proteins carry polymorphic C-terminal toxin domains (CdiA-CT), which are neutralized by specific CdiI immunity proteins to prevent self-inhibition. Here, we present the crystal structures of CdiA-CT,CdiI complexes from Klebsiella pneumoniae 342 and Escherichia coli 3006. The toxins adopt related folds that resemble the ribonuclease domain of colicin D, and both are isoacceptor-specific tRNases that cleave the acceptor stem of deacylated tRNAGAUIle. Although the toxins are similar in structure and substrate specificity, CdiA-CTKp342 activity requires translation factors EF-Tu and EF-Ts, whereas CdiA-CTEC3006 is intrinsically active. Furthermore, the corresponding immunity proteins are unrelated in sequence and structure. CdiIKp342 forms a dimeric β sandwich, whereas CdiIEC3006 is an alpha-solenoid monomer. Given that toxin-immunity genes coevolve as linked pairs, these observations suggest that the similarities in toxin structure and activity reflect functional convergence.