ORGANIZATION OF TELOMERIC AND SUBTELOMERIC CHROMATIN IN THE HIGHER-PLANT NICOTIANA-TABACUM

We have examined the structure and chromatin organization of telomeres in Nicotiana tabacum. In tobacco the blocks of simple telomeric repea ts (TTTAGGG)(n) are many times larger than in other plants, e.g., Arab idopsis thaliana or tomato. They are resolved as multiple fragments 60 -160 kb in size (in most cases 90-130 kb) on pulsed-field pi electroph oresis (PFGE) nf restriction endonuclease-digested DNA. The major subt elomeric repeat of the HRS60 family forms large homogeneous blocks of a basic 180 bp motif having comparable lengths. Micrococcal nuclease ( MNase) cleaves tobacco telomeric chromatin into subunits with a short repeat length of 157+/-5 bp; the subtelomeric heterochromatin characte rized by tandemly repeated sequences of the HRS60 family is cut by MNa se with a 180 bp periodicity. The monomeric and dimeric particles of t elomeric and subtelomeric chromatin differ in sensitivity to MNase tre atment: the telomeric particles are readily digested, producing ladder s with a periodicity of 7 bp, while the subtelomeric particles appear to be rather resistant to intranucleosomal cleavage. The results prese nted show apparent similarities in the organization of telomeric chrom atin in higher plants and mammals.