Internal micronuclear DNA regions which include sequences homologous to macronuclear telomeres are deleted during development in Tetrahymena

Abstract

C4A2 repeats are present in multiple clusters in both the macronucleus and micronucleus of Tetrahymena. Although the macronucleus is generated from the micronucleus after sexual conjugation, the repeats are telomeric sequences in the macronucleus but are internally located in the micronucleus (1). This study investigates the fate of the sequences adjacent to the micronuclear C4A2 repeats. Southern blot analyses of 21 C4A2-containing micronuclear clones show that extensive elimination of the adjacent sequences occurs during the formation of the macronucleus. Comparison of one C4A2-containing micronuclear clone with its derived macronuclear segment indicates that approximately 4.5 kb of DNA, which includes the C4A2 repeats and adjacent sequences on both sides is deleted from the macronucleus. The two regions adjoining the deletion are joined together to form a contiguous segment in the macronucleus. This excision of C4A2 repeats and surrounding sequences and the rejoining of the retained segments is probably the mechanism by which all or most of the other C4A2 adjacent sequences are eliminated.