EXPANDED CTG TRIPLET BLOCKS FROM THE MYOTONIC-DYSTROPHY GENE CREATE THE STRONGEST KNOWN NATURAL NUCLEOSOME POSITIONING ELEMENTS

Expanded blocks of repeating nucleotide triplets have been found in or near genes associated with several human diseases. In the case of myo tonic dystrophy, a block of repeating CTG trinucleotides is located do wnstream of the gene, and expansions of this block to repeats of n = 1 00 or more are frequently found in afflicted individuals. Using electr on microscopy, we recently demonstrated that these blocks form unusual ly stable nucleosomes. Here, competitive nucleosome reconstitution was employed to measure the energetics of nucleosome formation over CTG r epeat blocks of n = 75 and n = 130. These values were compared to the Xenopus borealis somatic 5S RNA gene, previously one of the strongest known natural nucleosome positioning elements. It is shown that DNA fr agments containing 75 and 130 CTG repeats are 6 and 9 times stronger i n nucleosome formation, respectively, than the 5S RNA gene. These find ings suggest that expanded CTG; blocks may profoundly alter local chro matin structure. (C) 1995 Academic Press, Inc.