Alternative model for chromatin organization of the Saccharomyces cerevisiae chromosomal DNA plasmid TRP1 RI circle (YARp1).

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TRP1 RI circle (now designated YARp1, yeast acentric ring plasmid 1) is a 1,453-base-pair artificial plasmid composed exclusively of Saccharomyces cerevisiae chromosomal DNA. It contains both the TRP1 gene and ARS1 (a DNA sequence that permits extrachromosomal maintenance of recombinant plasmids). This high-copy-number, relatively stable plasmid was shown to be organized into nucleosomes comparable to typical yeast chromatin, containing a possible maximum of nine nucleosomes per circle. Therefore, YARp1 can be used to examine the structure of chromatin of both a chromosomally derived replicator and a functional gene. By mapping regions of micrococcal nuclease cleavage in chromatin versus purified DNA, we located the positions of protected regions on the circle with reference to six unique restriction sites. Measurements made on patterns of early digestion products indicated that a region of approximately 300 base pairs in the vicinity of ARS1 was strongly resistant to micrococcal nuclease. The remainder of the plasmid appeared to be associated with five positioned nucleosomes and two nonnucleosomal, partially protected regions on the bulk of the molecules. After similar extents of digestion, naked DNA did not exhibit an equivalent pattern, although some hypersensitive cleavage sites matched sites found in the chromatin. These results are consistent with the interpretation that the protected domains are aligned with respect to a specific site or sites on the small circular chromatin.

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