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Prof. Patrick Pausch, Vilnius University: Of DNA making and RNA breaking in bacterial immunity

Speaker Series. Abstract: Reverse transcriptases (RTs) synthesize DNA from RNA templates. Prokaryotic RTs are linked to mobile genetic elements (MGE), either facilitating or interfering with their propagation. The ongoing conflict between viruses and their host has favoured the evolution of diverse RT-based systems that provide bacteria with immunity against phages, such as retrons, abortive phage infection systems (Abi), CRISPR-Cas RTs and unknown group (UG) systems. In this talk, I will present our recent efforts in understanding the retron system Eco2 (Ec67). How Eco2 mediates anti-phage defense by reverse transcribing non-coding RNA into multicopy single-stranded DNA (msDNA) is unknown. Unlike most other retrons, Eco2 employs a single protein that couples a reverse transcriptase (RT) and RNase H-like topoisomerase primase (TOPRIM). Cryo-EM reveals the unique homotrimeric nucleoprotein structure of Eco2, which supports an msDNA-dependent regulation of Eco2's RT-TOPRIM fusion protein. Purification of complex formation intermediates identifies the ribosomal protein S1 as a putative ncRNA-chaperone and translation regulator. Finally, we show that Eco2's broad defense against various phages is triggered by a phage-encoded endonuclease that degrades the msDNA. msDNA decay in turn activates the TOPRIM for tRNA cleavage, in turn halting protein production for abortive infection. These findings provide insights into the biogenesis, enzymatic regulation and immune defense mechanism of the single-protein retron system Eco2.

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