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. 2012;8(6):e1002771.
doi: 10.1371/journal.pgen.1002771. Epub 2012 Jun 21.

In vivo effects of histone H3 depletion on nucleosome occupancy and position in Saccharomyces cerevisiae

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In vivo effects of histone H3 depletion on nucleosome occupancy and position in Saccharomyces cerevisiae

Andrea J Gossett et al. PLoS Genet. 2012.

Abstract

Previous studies in Saccharomyces cerevisiae established that depletion of histone H4 results in the genome-wide transcriptional de-repression of hundreds of genes. To probe the mechanism of this transcriptional de-repression, we depleted nucleosomes in vivo by conditional repression of histone H3 transcription. We then measured the resulting changes in transcription by RNA-seq and in chromatin organization by MNase-seq. This experiment also bears on the degree to which trans-acting factors and DNA-encoded elements affect nucleosome position and occupancy in vivo. We identified ∼60,000 nucleosomes genome wide, and we classified ∼2,000 as having preferentially reduced occupancy following H3 depletion and ∼350 as being preferentially retained. We found that the in vivo influence of DNA sequences that favor or disfavor nucleosome occupancy increases following histone H3 depletion, demonstrating that nucleosome density contributes to moderating the influence of DNA sequence on nucleosome formation in vivo. To identify factors important for influencing nucleosome occupancy and position, we compared our data to 40 existing whole-genome data sets. Factors associated with promoters, such as histone acetylation and H2A.z incorporation, were enriched at sites of nucleosome loss. Nucleosome retention was linked to stabilizing marks such as H3K36me2. Notably, the chromatin remodeler Isw2 was uniquely associated with retained occupancy and altered positioning, consistent with Isw2 stabilizing histone-DNA contacts and centering nucleosomes on available DNA in vivo. RNA-seq revealed a greater number of de-repressed genes (∼2,500) than previous studies, and these genes exhibited reduced nucleosome occupancy in their promoters. In summary, we identify factors likely to influence nucleosome stability under normal growth conditions and the specific genomic locations at which they act. We find that DNA-encoded nucleosome stability and chromatin composition dictate which nucleosomes will be lost under conditions of limiting histone protein and that this, in turn, governs which genes are susceptible to a loss of regulatory fidelity.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Histone H3 depletion alters nucleosome occupancy genome-wide.
(A) Schematic representation of the strains used in this experiment. The H3 shutoff strain contains a deletion of one copy of histone H3 (HHT1) and a GAL1 promoter inserted upstream of the second copy (HHT2). (B) The average, Gaussian-smoothed dyad density for the wildtype and H3 shutoff strains at 0 and 3 hours were aligned on the +1 nucleosomes for 4,555 genes. Upon H3 depletion, the regular array of nucleosomes visible in both of the wildtype time points and the 0 hour H3 shutoff strain time point become disrupted, resulting in a loss of regular positioning of nucleosomes internal to the gene. (C) Log2 normalized average nucleosome occupancy in the wildtype strain versus the H3 shutoff strain. Genome-wide nucleosome occupancy in the two strains is similar prior to H3 depletion (r = 0.94) but decreases following H3 depletion (r = 0.81).
Figure 2
Figure 2. A subset of nucleosomes demonstrate behaviors specific to H3 depletion.
(A) Schematic representation of the nucleosome classification method. Nucleosomes were classified based on changes in center positions and changes in read center counts between 0 hours and 3 hours (Materials and Methods). (B) Classification of nucleosomes. The number of nucleosomes in each classification that occur in only the wildtype strain 3 hours after the shift to Galactose (black); only the H3 shutoff strain (white); and those classified identically in both strains (gray) is shown. Only the H3 shutoff-specific nucleosomes (white) were used in subsequent analyses. The total number of nucleosomes that could be classified in each category are as follows: wildtype, 43,508; H3 shutoff, 38,955; classified in both, 28,717.
Figure 3
Figure 3. Histone acetylation is associated with reduced nucleosome occupancy.
The positive (yellow) or negative (blue) association of previously published genome-wide data sets and genome annotations with nucleosome change categories (Materials and Methods). (A) Genome annotations, (B) histone modifications, (C) transcription-associated proteins, (D) chromatin remodelers and (E) histone occupancy. The number in parenthesis indicates the resolution of the detection platform in the data set used for comparison.
Figure 4
Figure 4. DNA sequence contributes to nucleosome occupancy changes following H3 depletion.
(A) In vivo nucleosome occupancy versus previously published in vitro nucleosome occupancy genome wide . The occupancy values were normalized as in , and log2 values are plotted. (B) Changes in the DNA sequence-based predicted occupancy (“model score” from [8]) at nucleosomes in the indicated categories at the 0 hour (dark gray) and 3 hour (light gray) nucleosome positions. The error bars indicate the standard error, and pairs marked with an * are significantly different at a p-value<0.001. (C) The degree of nucleosome occupancy reduction or retention following H3 depletion correlates with the DNA-sequence based predictions of change in nucleosome occupancy. Nucleosomes classified as “occupancy reduced” or “occupancy retained” are plotted. Because a position change is required for an alteration in predicted occupancy, nucleosomes classified as “position unchanged” were not included. pOcc = Occupancy prediction from ; Occ = occupancy in H3 shutoff strain.
Figure 5
Figure 5. Nucleosomes with reduced occupancy are overrepresented in promoters of genes with increased expression following H3 depletion.
The relationship between gene expression changes upon histone depletion (rows) and changes in nucleosome occupancy and position (columns). This relationship is shown for promoters, defined as the untranscribed region upstream of a gene, and genes, defined as the transcribed regions from .

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