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Bacterial DNA topoisomerase I can relax positively supercoiled DNA containing a single-stranded loop

1985; Elsevier BV; Volume: 185; Issue: 3 Linguagem: Inglês

10.1016/0022-2836(85)90075-0

1089-8638

Karla Kirkegaard, James C. Wang,

DNA and Nucleic Acid Chemistry

Using heteroduplex molecules formed from a pair of plasmids, one of which contains a small deletion relative to the other, it is shown that bacterial topoisomerase I can relax a positively supercoiled DNA if a short single-stranded loop is placed in the DNA. This result supports the postulate that the specificity of bacterial DNA topoisomerase I for negatively supercoiled DNA in its relaxation reaction derives from the requirement of a short single-stranded DNA segment in the active enzyme-substrate complex. Nucleolytic and chemical probing of complexes between bacterial DNA topoisomerase I and heteroduplex DNA molecules containing single-stranded loops ranging from 13 to 27 nucleotides in length suggests that the enzyme binds specifically to the region containing a single-stranded loop; the site of DNA cleavage by the topoisomerase appears to lie within the single-stranded loop, with the enzyme interacting with nucleotides on both sides of the point of cleavage.