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. Author manuscript; available in PMC: 2009 Dec 14.
Published in final edited form as: Cell. 2009 Feb 20;136(4):656–668. doi: 10.1016/j.cell.2009.01.045

Figure 2. Model of RNAi-Based Scanning and Sequence Elimination in Tetrahymena thermophila.

Figure 2

Simplified view of Twi1p scanning and targeting of sequence elimination. Nuclear development is not depicted (see Matzke and Birchler, 2005, for a detailed overview of nuclear progression during development). Each depicted cell is one of a mating pair (shaded cells).

(A) Generation of scan RNAs (scnRNAs) from the eliminated (green) and noneliminated (orange) germline sequences.

(B) Micronuclear export and loading of scnRNAs into cytoplasmic Twi1p.

(C) Twi1p import into the parental macronucleus, where Twi1p with bound scnRNAs “scans” the parental genome and are depleted from the population if matching to the parental, rearranged genome.

(D) Export of Twi1p, bound with eliminated-element-enriched scnRNAs, to the cytoplasm.

(E) Twi1p import into the developing macronucleus followed by targeting of elements to be eliminated with histone modifications (me, methylation).

(F) The newly rearranged macronuclear genome after recognition and elimination of elements targeted by the rearrangement machinery.