Transition between two forms of heterochromatin at plant subtelomeres

The manner of packing of the terminal DNA loci into nucleosomes and higher order structures may strongly influence their functional interactions. Besi des the structural flexibility of telomeric DNA sequences, conserved featur es of their chromatin including short nucleosome phasing (157 bp) and nucle osome sliding have been described previously. To gain a complementary knowl edge of subtelomeres, we have analysed the chromatin structure of two subte lomeric tandem repeats from the plant Silene latifolia: X43.1 and 15Ssp. X4 3.1 shows two distinct nucleosome periodicities - 157 and 188 bp. Preferred positions of its two nucleosomes have been mapped at both low and high res olution and the experimental results correspond to computer-predicted posit ions. 15Ssp is a newly-discovered sequence showing a telomere-associated po sition by PCR and a subtelomeric location by pulsed-field gel electrophores is and fluorescence in situ hybridisation. Its 159 bp sequence unit shows a tandem arrangement and the presence of micrococcal nuclease-hypersensitive sites when either naked DNA or chromatin is digested. Use of a chemical nu clease results in a regular nucleosome ladder of 157 bp periodicity. Moreov er, 15Ssp mononucleosomes show instability and absence of specific position ing, features typical for telomeric chromatin.