Transposon Invasion of the Paramecium Germline Genome Countered by a Domesticated PiggyBac Transposase and the NHEJ Pathway

Figure 5

Molecular mechanism of IES excision in Paramecium. The successive DNA intermediates that are formed during IES excision are displayed, with IESs in red and flanking MAC-destined DNA in black. The proteins that were shown to be required for proper IES excision are also represented. The first step is the introduction of 4-base staggered double-strand breaks at each IES end and depends on the PiggyMac domesticated transposase (Pgm). According to available knowledge of the classical NHEJ pathway in other organisms, a Ku70/Ku80 dimer is proposed to bind to each broken flanking DNA end and recruits the DNA-PKcs catalytic subunit. The last steps of the reaction were proposed to take place within a paired-end intermediate guided by annealing of the central TA present on each 5′ overhang [57]. The proteins involved in the removal of the 5′ terminal nucleotide have not been identified. For the 3′ processing step, the ligase IV is required for recruiting or activating a gap-filling DNA polymerase, which adds one nucleotide to the recessive end, prior to final ligation. A similar mechanism is proposed for the circularization of excised linear IES molecules (right part of the figure), providing that they are long enough. IES circles do not replicate and are actively degraded.