9vhl
cryoEM structure of retron-Eco7 complex (form II)
Structural highlights
FunctionRT78_ECOLX Reverse transcriptase (RT) component of antiviral defense system retron Ec78, composed of a non-coding RNA (ncRNA), this reverse transcriptase (RT), a probable ATPase and a putative HNH endonuclease. Expression of retron Ec78 confers protection against bacteriophage T5. At multiplicity of infection (MOI) of 0.02 cultures slow growth when infected with T5 but do not collapse, at MOI 2 cultures enter growth stasis (PubMed:33157039). Responsible for synthesis of msDNA-Ec78 (a linear ssDNA with a 5'-terminal phosphate residue). Unlike most known msDNAs the mature product does not have an RNA component. The retron transcript serves as primer and template for the reaction, and codes for the RT. Not mutagenic when cloned in E.coli (PubMed:9281493). It is thought to be synthesized as a branched RNA with a 2',5'-phosphodiester linkage to ssDNA; the linkage is cleaved endonucleolytically by ExoVII (xseA-xseB) leaving the observed mature 5'-ssDNA terminus (Probable) (PubMed:26626352). Overexpression of the ncRNA and RT, which leads to increased levels of msDNA, is not mutagenic in vivo. As the stem in the msDNA does not have a mismatch it probably does not bind or sequester MutS and/or MutL (PubMed:7885227).[1] [2] [3] [4] [5] Publication Abstract from PubMedRetrons are bacterial antiphage defense systems comprising a reverse transcriptase (RT), a non-coding RNA (ncRNA), and cognate effector proteins. The RT synthesizes multicopy single-stranded DNA (msDNA) from the ncRNA template to detect phage invasion. This study focuses on Retron-Eco7, which integrates retron-based sensing with the effector module of Septu-a characterized antiphage system in which the PtuAB complex mediates nuclease-dependent defense. However, the activation mechanism of this hybrid system remains unclear. Here, we determined cryo-electron microscopy structures of the RT-msDNA-PtuAB quaternary complex and the PtuAB binary complex in Retron-Eco7. Structural analyses reveal that the DNA stem-loop of msDNA extensively interacts with PtuA subunits via electrostatic interactions. We establish Retron-Eco7 as a novel toxin-antitoxin system, in which RT-msDNA acts as the antitoxin, directly binding and neutralizing the PtuAB toxin. Furthermore, we identified a phage-encoded flap endonuclease as a trigger for Retron-Eco7 activation, which cleaves msDNA to release the PtuAB toxin. Our findings demonstrate the diversity in bacterial retron defense systems and uncover a novel activation mechanism of the Septu-derived retron toxin-antitoxin system. Phage nuclease-mediated defense activation of the bacterial Retron-Eco7 toxin-antitoxin system.,Dai Z, Liu C, Wang Y, Chen X, Fu X, Yang K, Zhu R, Jia X, Chen Y, Tao P, Guan Z, Zou T Nucleic Acids Res. 2025 Nov 13;53(21):gkaf1173. doi: 10.1093/nar/gkaf1173. PMID:41277685[6] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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