Developmental precise excision of Oxytricha trifallax telomere-bearing elements and formation of circles closed by a copy of the flanking target duplication.

The 4.1 kbp TBE1 elements of Oxytricha fallax and Oxytricha trifallax are deduced to transpose into a centrisymmetric target, CAnTG, and to duplicate the central AnT. Despite conserved C(A4C4)2 telomeric repeats at their tips, free TBE1s found during macronuclear development are not linear but 4.1 kbp circles closed on one copy of the AnT target duplication. The macronucleus-destined flanks are rejoined to regenerate the target, effecting efficient and precise somatic reversion of the germline transpositional mutation. A model is presented in which transposase catalyzes concerted precise rejoining of the flanks and cyclization of the excised element.