6PFQ:
reference: Structural basis for the evolution of cyclic phosphodiesterase activity in the U6 snRNA exoribonuclease Usb1., Nomura Y, Montemayor EJ, Virta JM, Hayes SM, Butcher SE, Nucleic Acids Res. 2020 Feb 20;48(3):1423-1434. doi: 10.1093/nar/gkz1177. PMID: 31832688

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