Abstract
SummaryUsing a genetic model, we present a high-resolution chromatin fiber analysis of transcriptionally active (Xa) and inactive (Xi) X chromosomes packaged into euchromatin and facultative heterochromatin. Our results show that gene promoters have an open chromatin structure that is enhanced upon transcriptional activation but the Xa and the Xi have similar overall 30 nm chromatin fiber structures. Therefore, the formation of facultative heterochromatin is dependent on factors that act at a level above the 30 nm fiber and transcription does not alter bulk chromatin fiber structures. However, large-scale chromatin structures on Xa are decondensed compared with the Xi and transcription inhibition is sufficient to promote large-scale chromatin compaction. We show a link between transcription and large-scale chromatin packaging independent of the bulk 30 nm chromatin fiber and propose that transcription, not the global compaction of 30 nm chromatin fibers, determines the cytological appearance of large-scale chromatin structures.
Highlights
Chromatin structure modulation is central to the control of gene expression (Cairns, 2009; Li et al, 2007)
Gene Promoters Have an Open Chromatin Structure Gene-rich regions of the human genome are enriched in ‘‘open’’ 30 nm chromatin fibers (Gilbert et al, 2004). To map these 30 nm chromatin fibers at high resolution, we developed our previously described approach to fractionate chromatin fibers based on their secondary structure and hybridized them to Illumina SNP arrays
To separate 30 nm chromatin fragments based on their conformation, we centrifuged them in a continuous sucrose gradient (6%–40%) under physiological salt conditions (80 mM NaCl)
Summary
We present a high-resolution chromatin fiber analysis of transcriptionally active (Xa) and inactive (Xi) X chromosomes packaged into euchromatin and facultative heterochromatin. Our results show that gene promoters have an open chromatin structure that is enhanced upon transcriptional activation but the Xa and the Xi have similar overall 30 nm chromatin fiber structures. The formation of facultative heterochromatin is dependent on factors that act at a level above the 30 nm fiber and transcription does not alter bulk chromatin fiber structures. Largescale chromatin structures on Xa are decondensed compared with the Xi and transcription inhibition is sufficient to promote large-scale chromatin compaction. We show a link between transcription and largescale chromatin packaging independent of the bulk 30 nm chromatin fiber and propose that transcription, not the global compaction of 30 nm chromatin fibers, determines the cytological appearance of large-scale chromatin structures
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