Coralie Zangarelli

Abstract 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.

Highlights • A DNA-binding-deficient Ku mutant allows programmed DNA cleavage in Paramecium • In the repair-deficient cells, broken DNA ends are trimmed or healed by telomere addition • Uncoupling DNA cleavage and repair favors chromosome translocations • Tight coupling ensures faithful genome assembly during programmed rearrangements Summary In the ciliate Paramecium, precise excision of numerous internal eliminated sequences (IESs) from the somatic genome is essential at each sexual cycle.

Abstract Ciliates undergo developmentally programmed genome elimination, in which small RNAs direct the removal of target DNA segments, including transposable elements. At each sexual generation, the development of the macronucleus (MAC) requires massive and reproducible elimination of a large proportion of the germline micronuclear (MIC) genome, leading to a highly streamlined somatic MAC genome.