The Pnkp-Hen1 RNA repair pathway confers protective immunity to recurrent RNA damage. (A) The adjacent co-oriented ORFs encoding the Hen1 and Pnkp polypeptides comprise a bacterial RNA repair cassette conserved in taxa from 10 different bacterial phyla. Pnkp is a trifunctional RNA repair enzyme composed of 5′-OH polynucleotide kinase, 2′,3′ phosphatase, and ligase domains. Hen1 is composed of a C-terminal methyltransferase catalytic domain (MTase) fused to a unique N-terminal module that binds to the Pnkp ligase domain. (B) 2′,3′ cyclic phosphate and 5′-OH ends are substrates for healing and sealing by Pnkp and Hen1. The 5′ end is phosphorylated by the Pnkp kinase and the 2′,3′ cyclic phosphate is removed by the Pnkp phosphatase. The Hen1 methyltransferase installs a 2′-OCH3 mark at the terminal ribonucleoside prior to ligation of the ends. The ligation reaction is mediated by a heterodimer of the Pnkp ligase domain and the Hen1 N-terminal domain. The repair junction with the methyl mark is then resistant to scission by transesterifying endoribonucleases.
