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Structure-functional characterization of Lactococcus AbiA phage defense system.

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Bacterial abortive infection system (Abi) reverse transcriptases (RTs) defend against phages. This study reveals the Lactococcus AbiA structure, showing it forms a dimer and requires its HEPN domain for DNA synthesis, distinct from other Abi RTs.

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Area of Science:

  • Structural Biology
  • Enzymology
  • Bacteriology

Background:

  • Bacterial reverse transcriptases (RTs) are diverse enzymes, with abortive infection (Abi) RTs like AbiA, AbiK, and Abi-P2 providing defense against bacteriophages.
  • Abi RTs uniquely synthesize DNA products randomly, independent of templates or primers.
  • Structural data for AbiA was previously lacking, unlike AbiK and Abi-P2.

Purpose of the Study:

  • To determine the crystal structure of Lactococcus AbiA polymerase.
  • To elucidate the structural and functional characteristics of AbiA, including its oligomeric state and domain composition.
  • To investigate the role of its domains in DNA polymerase activity and in vivo phage defense.

Main Methods:

  • X-ray crystallography was used to determine the structure of Lactococcus AbiA.
  • In vitro DNA polymerase assays were performed to assess AbiA's enzymatic activity.
  • Bacteriophage infection assays were conducted in vivo to validate the findings.

Main Results:

  • The crystal structure of AbiA revealed a three-domain architecture: an RT-like domain, a helical domain, and a higher eukaryotes and prokaryotes nucleotide-binding (HEPN) domain.
  • AbiA forms a dimer, contrasting with the trimeric/hexameric structures of AbiK and Abi-P2.
  • In vitro assays confirmed AbiA's DNA polymerase activity, which necessitates the HEPN domain, despite its enzymatic inactivity.

Conclusions:

  • The structural and biochemical data provide new insights into AbiA function and its distinction from other Abi RTs.
  • AbiA's dimeric structure and HEPN domain requirement are key features of its DNA synthesis mechanism.
  • In vivo phage defense mediated by AbiA may not involve AbiA-induced cell death, suggesting alternative defense pathways.