Alternative Processing of Sequences During Macronuclear Development inTetrahymena thermophila1

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Abstract
DNA is eliminated during development of the somatic MACronucleus from the germinal MICronucleus in the ciliated protozoan, Tetrahymena thermophila. Facultatively persistent sequences are a class of sequences that persist in the MAC DNA of some cell lines but are eliminated from the MAC DNA of other cell lines. One cloned MAC fragment contains a persistent sequence as well as sequences normally retained in the MAC. When this cloned fragment was used to construct MAC restriction maps of this region in cell lines whose MAC DNAs do, or do not, contain the persistent sequence, extensive variation in the map flanking this region was observed. The different DNA rearrangement of this MIC segment are epigenetically determined during or soon after MAC development. Moreover, different rearrangements may occur among the 45 copies of this MIC segment as a MAC is formed, resulting in polymorphisms that are later resolved by phenotypic assortment.