2 TYPES OF TELOMERIC CHROMATIN IN TETRAHYMENA-THERMOPHILA

The telomeric d(GGGGTT) (.) d(AACCCC) repeat tracts (G(4)T(2) repeats) in Tetrahymena theumophila macronuclei were shown previously to be pa ckaged in a non-nucleosomal DNA-protein complex. Here, we demonstrate that these telomeric repeats, together with a short region of the imme diately adjacent non-telomeric sequence, exist in two distinct types o f chromatin. The non-nucleosomal complex (type I complex) comprises si milar to 90 to 97% of telomeric DNA, has no apparent underlying period ic nucleosomal substructure, and includes the whole telomeric tract as well as the immediately adjacent sequence. Type II chromatin, compris ing the remaining similar to 3 to 10% of the total telomeric DNA, cons ists of tightly packed nucleosomes clustered at the inner border of th e telomeric tracts, with a periodicity of 154(+/-3) bp. This packing i s similar to that of telomeric nucleosomes in vertebrates. However, in contrast to the unstability of vertebrate telomeric mononucleosomes, the T. theumophila mononucleosomes were stable to micrococcal nuclease digestion. During the natural lengthening of the T. thermophila telom eric DNA tracts that occurs in vegetatively dividing cells, the overal l 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 o ne, up to three to four, per telomeric tract. This finding of telomeri c nucleosomes in T. thermophila suggests that the difference between v ertebrates and lower eukaryotes in telomeric chromatin structure is qu antitative rather than qualitative. We propose that deposition of nucl eosomes competes with non-nucleosomal complex formation on telomeric D NA, resulting in a sub-population of chimeric telomeres containing inn er nucleosomes abutting a distal, variable length of type I complex. : (C) 1998 Academic Press.