An unusual sequence arrangement in the telomeres of the germ-line micronucleus in Tetrahymena thermophila

Abstract

The ciliated protozoan Tetrahymena thermophila contains two nuclei that differ dramatically in function, chromosome size and number, chromatin structure, and mode of division. It is possible that the telomeres of the two nuclei have different functions. Although macronuclear telomeric DNA has been well characterized and consists of tandem G4T2/C4A2 repeats that are synthesized by the enzyme telomerase, micronuclear telomeres have not been isolated previously. Here, we report the identification and cloning of micronuclear telomeres and demonstrate that although they contain the same terminal tandem G4T2 repeats as macronuclear telomeres, they are strikingly different in three respects. First, the tracts of G/C-rich telomeric repeats are approximately seven times longer in the micronucleus than in the macronucleus (approximately 2.0-3.4 vs. approximately 0.3-0.5 kb, respectively) from the same cell population. Second, the immediate telomere-associated sequences (TASs) from six different micronuclear chromosome ends have an unusually high G/C content and degree of homology to one another, unlike macronuclear TASs. The TAS from at least one micronuclear chromosome is unique to micronuclear telomeres and is not present in the macronucleus. Finally, and unexpectedly, all micronuclear telomere clones contain an inner homogeneous tract of a variant G4T3 repeat adjacent to the distal tract of G4T2 repeats. The native micronuclear telomeric DNA is composed of approximately 30% G4T3 repeats, corresponding to 0.6-1.0 kb per average telomere, positioned centromere-proximally to most or all of the G4T2 repeats. Neither the G4T3 sequence nor any other variant repeat is found in macronuclear telomeres. Furthermore, such a homogeneous tract of a variant repeat has not been found in the telomeres of any eukaryote.