Two types of telomeric chromatin in Tetrahymena thermophila

The telomeric d(GGGGTT).d(AACCCC) repeat tracts (G4T2 repeats) in Tetrahymena thermophila macronuclei were shown previously to be packaged in a non-nucleosomal DNA-protein complex. Here, we demonstrate that these telomeric repeats, together with a short region of the immediately adjacent non-telomeric sequence, exist in two distinct types of chromatin. The non-nucleosomal complex (type I complex) comprises approximately 90 to 97% of telomeric DNA, has no apparent underlying periodic nucleosomal substructure, and includes the whole telomeric tract as well as the immediately adjacent sequence. Type II chromatin, comprising the remaining approximately 3 to 10% of the total telomeric DNA, consists of tightly packed nucleosomes clustered at the inner border of the telomeric tracts, with a periodicity of 154(+/-3) bp. This packing is similar to that of telomeric nucleosomes in vertebrates. However, in contrast to the unstability of vertebrate telomeric mononucleosomes, the T. thermophila mononucleosomes were stable to micrococcal nuclease digestion. During the natural lengthening of the T. thermophila telomeric DNA tracts that occurs in vegetatively dividing cells, the overall ratio of type I and type II chromatin did not change. However, type I complex expanded with the length of the telomeric DNA repeat tract, and the number of telomeric nucleosomes increased from an average of one, up to three to four, per telomeric tract. This finding of telomeric nucleosomes in T. thermophila suggests that the difference between vertebrates and lower eukaryotes in telomeric chromatin structure is quantitative rather than qualitative. We propose that deposition of nucleosomes competes with non-nucleosomal complex formation on telomeric DNA, resulting in a sub-population of chimeric telomeres containing inner nucleosomes abutting a distal, variable length of type I complex.