Abstract
Chromatin plays a pivotal role in genome expression, maintenance, and replication. To better understand chromatin organization, we developed a novel proximity-tagging method which assigns unique DNA barcodes to molecules that associate in 3D space. Using this method – Proximity Copy Paste (PCP) – we mapped the connectivity of individual nucleosomes in Saccharomyces cerevisiae. By analyzing nucleosome positions and spacing on single molecule fibers, we show that chromatin is predominantly organized into regularly spaced nucleosome arrays that can be positioned or delocalized. Basic features of nucleosome arrays are generally explained by gene size and transcription. PCP can also map long-range, multi-way interactions and we provide the first direct evidence supporting a model that metaphase chromosomes are compacted by cohesin loop clustering. Analyzing single-molecule nuclease footprinting data we define distinct chromatin states within a mixed population to show that non-canonical nucleosomes, notably Overlapping-Di-Nucleosomes (OLDN) are a stable feature of chromatin. PCP is a versatile method allowing the detection of the connectivity of individual molecules locally and over large distance to be mapped at high-resolution in a single experiment.
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