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 repeat s 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 du plication. 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 trans posase catalyzes concerted precise rejoining of the flanks and cycliza tion of the excised element.