Hyperacetylated nucleosomes

MCF-7 cells exposed to 50 nM TSA were in G1 phase (>70%). However, cells treated with 1 J,M TSA arrested cells predominantly in G2/M phase, suggesting a dose-dependent fashion. With either dose, cells accumulated least in the S phase. In addition to accumulation of hyperacetylated nucleosome core histones, TSA enhanced p21 expression. Therefore, flow cytometry (FACS) offers an efficient tool for analyzing the action of HDACIs in cell proliferation. This method is particularly useful for application to clinical samples, where cell numbers may be small. The effects of HDACIs on the cell cycle are dose-dependent. 

But what should happen upon histone hyperacetylation if nucleosomes, on average, do not overtwist DNA Then no change in (ALkn) should be observed. 

Fig. 11. Effects of histone acetylation on the folding and stability of the nucleosome core particle. A. NaCl dependence of the sedimentation coefficient (s2o,w) of the nucleosome core particles with different extent of acetylation soo D] [379]. B. Dose-response curves obtained with hypo- ( ) and hyperacetylated...

B. Effect of the ionic strength on hyperacetylated 208-12 nucleosome arrays as visualized by electron microscopy. The numbers to the left indicate the milimolar NaCl concentration [369]. [Reproduced from Garcia-Ramirez M. et al. (1995) J. Biol. Chem. 270, 17923-17928, with permission from The American Society for Biochemistry and Molecular Biology.]... 

Oliva, R., Bazett-Jones, D.P., Locklear, L., and Dixon, G.H. (1990) Histone hyperacetylation can induce unfolding of the nucleosome core particle. Nucleic Acids Res. 18(9), 2739-2747. 

Subsaturated reconstituted nucleosomal arrays from 208-12 DNA fragment and HeLa control or hyperacetylated core histones... 

Several reports have used AFM imaging, in conjunction with traditional biochemical methods, to address this question (it must be noted that an in-depth AFM study of this issue is still to be performed). The AFM data suggest thus far that hyperacetylated histones isolated from cells treated with histone deacetylase inhibitors produce beads-on-a string structures somewhat more extended than those obtained using histones purified from control cells [37,38], in agreement with EM imaging results obtained on circular chromatin templates [49]. In addition, acetylation seems to enhance the non-random nucleosome-loading behavior seen in subsaturated nucleosomal arrays (see above, and Table 1). 

What is the relevance of these observations to transcriptional control If, indeed, the tails become more loosely associated with DNA upon hyperacetylation, it is possible that the underlying DNA becomes more accessible to nonhistone regulators. In vitro experiments with purified chromatin components and particular transcriptional regulators (A. Wolffe, J. Workman, and their colleagues) have found that histone hyperacetylation potentiates binding to nucleosomal substrates by such proteins as TFIIIA, Gal4, and USF. Whether such potentiation of binding occurs in vivo is unknown. 

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