Structural features of rat liver telomere oligonucleosomes.

Telomeres are specialized structures found at ends of eukaryotic chromosomes. Investigating telomere structure may lead to a better understanding of how terminal nucleoproteins might affect (1) the expression of nearby genes, (2) the localization of telomeres within the nucleus, and (3) the accessibility of this region to proteins involved with telomere metabolism including maintenance, replication, recombination, and repair. In this study classical biochemical and biophysical techniques were used to probe for this structure in rat liver cells and address four questions: (1) Do the telomere subunits generated by micrococcal nuclease digestion have similar physical (hydrodynamic) properties as bulk chromatin; (2) does such chromatin condense into higher-order structure similar to that of bulk chromatin; (3) is histone H1 associated with such chromatin; and (4) are there any unusual base modifications associated with telomeric DNA? Nuclease digested telomere subunits do indeed sediment like bulk nucleosomes. In addition, sedimentation analysis shows that telomeric nucleosome arrays condense with increasing ionic strength and molecular weight in a manner comparable to that of bulk chromatin despite the very short repeat length. Frictional coefficient calculations are consistent with greater compaction of the telomere chromatin. The condensed telomere nucleosome arrays do not exhibit the $\sim$100 bp DNase II repeat characteristic of condensed bulk chromatin. Telomeric dinucleosomes contain histone H1 as determined by nucleoprotein gel electrophoresis. \sp{32}P-post labeling in combination with two-dimensional thin layer chromatography of purified telomere DNA ($>$90% purity) shows that several modified bases known to be either present in subtelomeric DNA of other organisms or involved with modulating secondary structure were absent in such DNA. However, methylated cytosines may be present in telomeric DNA at levels comparable to that of bulk. These results indicate that a major fraction of the long telomeres of rat are organized as specialized nucleosome arrays with features similar, but not identical to those of bulk chromatin.Ph.D.Cellular and Molecular BiologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/105068/1/9635485.pdfDescription of 9635485.pdf : Restricted to UM users only