AN UNUSUAL SEQUENCE ARRANGEMENT IN THE TELOMERES OF THE GERM-LINE MICRONUCLEUS IN TETRAHYMENA-THERMOPHILA

The ciliated protozoan Tetrahymena thermophila contains two nuclei tha t differ dramatically in function, chromosome size and number, chromat in structure, and mode of division. It is possible that the telomeres of the two nuclei have different functions. Although macronuclear telo meric DNA has been well characterized and consists of tandem G(4)T(2)/ C(4)A(4) repeats that are synthesized by the enzyme telomerase, micron uclear telomeres have not been isolated previously. Here, we report th e identification and cloning of micronuclear telomeres and demonstrate that although they contain the same terminal tandem G(4)T(2) repeats as macronuclear telomeres, they are strikingly different in three resp ects. First, the tracts of G/C-rich telomeric repeats are approximatel y seven times longer in the micronucleus than in the macronucleus (sim ilar to 2.0-3.4 vs. similar to 0.3-0.5 kb, respectively) from the same cell population. Second, the immediate teIomere-associated sequences (TASs) from six different micronuclear chromosome ends have an unusual ly high G/C content and degree of homology to one another, unlike macr onuclear TASs. The TAS from at least one micronuclear chromosome is un ique to micronuclear telomeres and is not present in the macronucleus. Finally, and unexpectedly, all micronuclear telomere clones contain a n inner homogeneous tract of a variant G(4)T(3) repeat adjacent to the distal tract of G(4)T(3) repeats. The native micronuclear telomeric D NA is composed of approximately 30% G(4)T(3) repeats, corresponding to O.6-1.0 kb per average telomere, positioned centromere-proximally to most or all of the G(4)T(2) repeats. Neither the G(4)T(3) sequence nor any other variant repeat is found in macronuclear telomeres. Furtherm ore, such a homogeneous tract of a variant repeat has not been found i n the telomeres of any eukaryote.