Supercoiled recombinant DNAs containing the human adult a-globin gene region have been probed with nuclease SI in vitro. While agarose gel electrophoresis showed only one predominant, double-stranded cleavage generated by SI within 6 kb of human DNA and 4 kb of pBR322 sequence, a high resolution gel analysis reveals that the unique Sl-hypersensitive locus in the human adult a-globin gene region actually contains more than 15 authenti SI cleavage sites closely spaced together. The mapping approach used here locates the specific SI cleavage sites on both DNA strands at the nucleotide sequence level. Interestingly, most of these sites are mapped within a 90 bp stretch of GC-rich (66%) polypyrimidine'polypurine DNA that is located 1060 to 1150 bp upstream from the al-globin gene. These results provide the first high resolution map of double-stranded Sl-cleavage sites induced within a specific DNA sequence under supercoil strain. The distribution and relative cutting frequencies of these sites mapped are consistent with a slippage mechanism in which the simple repeating sequences are organized into base-mismatched duplex on supercoiled DNA.
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