Exposure of HeLa cells to Na butyrate leads to an accumulation of multiacetylated forms of histones H3 and 14. Our studies of histone acetylation in HeLa S-3 cells show that 7 mM butyrate suppresses the deacetylation of histones without influencing the rate of radioactive acetate incorporation. An alteration in nucleosome structure in highly acetylated chromatin is indicated by an increased rate of DNA After the discovery of histone acetylation in 1964 (1), one of us suggested (2, 3) that this post-synthetic modification of histone structure could provide an enzymatic mechanism for modulating the interactions between histones and DNA in ways that affect the structure and function of chromatin. Numerous correlations have been noted between increased acetylation of the histones and gene activation for RNA synthesis (for recent reviews, see refs. 4-6). Acetylation of the lysine residues in the basic amino-terminal regions of the histones neutralizes their positive charges and would be expected to weaken their interactions with the phosphate groups of the DNA strand enveloping the nucleosome core.In order to relate changes in histone acetylation to chromatin structure and function, we have investigated the effects of acetylation on the nuclease sensitivity of associated DNA sequences. This approach is based on previous observations that transcriptionally active DNA sequences are preferentially degraded during limited digestions with DNase I, but not by digestion with staphylococcal nuclease (7,(8)(9)(10)(11). Highly acetylated chromatin has been obtained by the exposure of HeLa S-3 cells to 7 mM Na butyrate (12). We find that the accumulation of the multiacetylated forms of histones H3 and H4 under these conditions is due to a suppression of histone deacetylase activity. A consequence of increased histone acetylation is a more rapid rate of degradation of the associated DNA sequences during limited digestions with DNase I. In HeLa cells and in avian erythrocytes, the multiacetylated forms of histones H3 and H4 are preferentially released during limited DNase I digestions,

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A yeast gene encoding a DNA-binding protein that recognizes the telomeric repeat sequence TTAGGG found in multicellular eukaryotes was identified by screening a Agtll expression library with a radiolabeled TTAGGG multimer. This gene, which we refer to as TBFI (TTAGGG repeat-binding factor 1), encodes a polypeptide with a predicted molecular mass of 63 kDa. The TBF1 protein, produced in vitro by transcription and translation of the cloned gene, binds to (TTAGGG) Among all eukaryotes examined to date, the telomere is a highly conserved structure designed to protect chromosomes from degradation and fusion (for reviews, see references 6 and 32). Telomeres are composed of multiple repeats of short sequence elements (typically 5 to 8 bp in length, with a GT-rich strand oriented 5' to 3' toward the end of the chromosome) and range in length from a few repeat units to >10 kb. The repeated sequence (TTAGGG)n is found at telomeres in all vertebrates, certain slime molds, and trypanosomes; (TTGGGG)n and (TlTITGGGG)n are found in the ciliated protozoan Tetrahymena and Oxytricha species, respectively; and (TG1_3)n is found in the yeast Saccharomyces cerevisiae. In organisms whose telomeres have been examined in detail, the GT strand extends 12 to 16 nucleotides (two repeats) beyond the complementary C-rich strand.Proteins (5,8,19). Biochemical (11, 31) and cytological (17) studies have shown that RAP1 is bound to these sites in vivo. Furthermore, mutations in RAP1 affect telomere length, clearly establishing a role for RAP1 in the regulation of telomere structure (11,22,29

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