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

Grant C Gucinski1, Karolina Michalska2, Fernando Garza-Sánchez3

  • 1Biomolecular Science and Engineering Program, University of California, Santa Barbara, CA, USA.

Structure (London, England : 1993)
|September 14, 2019
PubMed

Insights

Contact-dependent growth inhibition (CDI) toxins from Klebsiella pneumoniae and Escherichia coli cleave tRNA, but differ in activity requirements. Their unrelated immunity proteins suggest functional convergence despite similar toxin structures.

Area of Science:

  • Bacteriology
  • Structural Biology
  • Molecular Biology

Background:

  • Contact-dependent growth inhibition (CDI) is mediated by CdiB-CdiA secretion systems for interbacterial competition.
  • CdiA effector proteins contain C-terminal toxin domains (CdiA-CT) neutralized by CdiI immunity proteins.

Purpose of the Study:

  • To elucidate the structural and functional mechanisms of CdiA-CT toxins and their cognate CdiI immunity proteins.
  • To investigate the evolutionary relationship between toxin structure, activity, and immunity.

Main Methods:

  • Crystal structure determination of CdiA-CT-CdiI complexes from Klebsiella pneumoniae 342 and Escherichia coli 3006.
  • Biochemical assays to determine substrate specificity and activity requirements of the toxins.

Main Results:

  • The toxins share structural similarity with colicin D ribonuclease domains and function as isoacceptor-specific tRNases, cleaving deacylated tRNAIle.
  • CdiA-CTKp342 requires translation factors EF-Tu and EF-Ts for activity, while CdiA-CTEC3006 is intrinsically active.
  • CdiIKp342 and CdiIEC3006 exhibit unrelated structures (dimeric β-sandwich vs. α-solenoid monomer), despite co-evolving with their respective toxins.

Conclusions:

  • The conserved toxin structure and substrate specificity suggest functional convergence in CDI systems.
  • Divergent immunity protein structures indicate distinct evolutionary paths for neutralization mechanisms within CDI systems.
Keywords:
bacterial competitiontoxin-antitoxin systemstwo-partner secretiontype V secretion system

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