A development-specific POLX essential for programmed genome rearrangement in Paramecium tetraurelia

During the sexual cycle, programmed genome rearrangement (PGR) in Paramecium tetraurelia involves the non-homologous end joining (NHEJ) DNA repair pathway to eliminate specific germinal Internal Eliminated Sequences (IESs) from the newly developing somatic nucleus. In addition to the core NHEJ factors Ku70/80 and Xrcc4/Lig4, additional enzymes are required to process the 4-base 5’-protruding ends generated following DNA cleavage at IES boundaries, prior to their ligation. Here, we report that PolX (a,b,c,d), four P. tetraurelia distant orthologs of the human Polλ DNA polymerase, are involved in repair of IES excision junctions. During rearrangements, PolX-depleted cells accumulate genome-wide errors, such as unrepaired double-strand breaks, 1-nucleotide deletions and IES retention. Although all PolX paralogs can process DNA ends, two of them (PolXa&b) are induced during PGR and have acquired tight nuclear anchoring properties through their N-terminal region, which contains a predicted BRCT domain. Finally, we show that PolXa accumulates in nuclear foci together with other NHEJ proteins and the Dicer-like enzyme Dcl5, which is involved in the biogenesis of IES-specific small RNAs. We propose that these “DNA repair foci” correspond to the sites where IES concatemers, a by-product of IES excision, are ligated together to produce the precursors of iesRNAs.