A developmentally eliminated sequence in the flanking region of the histone H1 gene in Tetrahymena thermophila contains short repeats.

In Tetrahymena, as in other ciliated protozoans, a transcriptionally active, 'somatic' macronucleus develops from a transcriptionally inactive 'germline' micronucleus after conjugation. The process of development involves elimination of germline DNA segments at thousands of locations in the genome. The characterization of one of these segments in Tetrahymena thermophila is described here. This micronucleus-specific DNA has been identified by comparing the sequence of the corresponding micronuclear and macronuclear regions. The micronucleus-specific DNA is over 1 kb long, is AT-rich and has TTT direct repeats at its termini. At one end of the micronuclear sequence there is a 130 bp duplication, and at the other end there are several related repeats of a 13-mer. Short G-rich sections are found in the middle of the eliminated DNA, as well as on one side of the rearrangement junction. Short G-rich segments are also detectable in three previously described micronucleus-specific sequences. The micronuclear sequence described here is a member of a repeat family. Cross-hybridizing sequences are also detectable in some other Tetrahymena species. The distribution of cross-hybridizing sequences among related species is not consistent with the phylogenetic tree.