NUCLEOSOME ASSEMBLY ON TELOMERIC SEQUENCES

The organization of telomeric chromatin differs from that of bulk chro matin in some peculiar features, such as the unusually short nucleosom al spacing found in vertebrates. Telomeric DNAs are straight, since th ey consist mostly of 6-8-bp repeated sequences, therefore out of phase with the B DNA period. This feature should be of relevance in nucleos ome formation, suggesting the usefulness of studying simple model syst ems of nucleosome assembly. We reconstituted nucleosomes in vitro, by using purified histone octamers and/or by octamer transfer from chicke n erythrocyte nucleosomes, onto telomeric sequences from human, Arabid opsis thaliana, and Saccharomyces cerevisiae. All of these telomeres c ontain GGG and GGT triplets but are characterized by different repeat lengths (6, 7, and 8 bp). The free energies involved in the associatio n process are the highest among the biological sequences so far assaye d, suggesting a main role of DNA flexibility in the assembly of telome ric chromatin. Digestion studies with DNase I, hydroxyl radicals, exon uclease III, and lambda exonuclease indicate that telomeric nucleosome s are characterized by multiple translational positioning without rota tional phasing, whereas the telomeric DNA folding around the histone o ctamer shows the canonical periodicity of about 10.2 bp. The experimen tal results and a theoretical simulation of DNase I digestion indicate a multiple nucleosome positioning with the periodicity of telomeric D NA. This suggests a main role of local chemical recognition between te lomeric sequences and the histone octamer in nucleosome assembly.