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Example sentences for: supercoils
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The number of potential Zn(II) binding cysteine motifs range from none in S. cerevisiae DNA topoisomerase III to four highly conserved tetracysteine motifs in the beta family of the topoisomerase III enzymes [ 38 ] . The Zn(II) domain formed by these tetracysteine motifs may be required for interaction with single-strand DNA in removal of hypernegative supercoils [ 39 ] or disruption of early recombination intermediates between inappropriately paired DNA molecules [ 40 ] .
However, the resulting relaxation activity is much less efficient than that of the intact enzyme, suggesting that coordinated actions of the two domains are required for efficient removal of negative supercoils from DNA.
Analysis of the time course of relaxation with 6 pmoles of topoisomerase I or top67 reconstituted with ZD (Figure 2b) showed that negative supercoils were removed at a much slower rate by the reconstituted activity.
Escherichia coli DNA topoisomerase I is a representative example of type IA DNA topoisomerase (for reviews, see refs [ 1 2 ] ). Its major biological role in the bacterial cell is the removal of excessive negative supercoils from DNA to maintain the DNA at optimal superhelical density along with DNA gyrase [ 3 ] . The enzyme has a molecular weight of 97 kDa and the active site tyrosine responsible for DNA cleavage is found in the 67 kDa N-terminal transesterification domain.
Removal of negative supercoils from DNA by bacterial type IA topoisomerase involves the following steps: (1) binding of the enzyme to the junction of double-stranded and single-stranded DNA [ 8 ] ; (2) cleavage of a single-strand of DNA near the junction with cleavage sequence preference of a cytosine in the -4 position to form the covalent intermediate [ 9 10 ] ; (3) conformational change of the covalent enzyme-DNA complex to result in physical separation of the 5' phosphate covalently linked to the active tyrosine, and the 3' hydroxyl of the cleaved DNA; (4) passage of the complementary single strand through the break; (5) enzyme conformational change to bring the 5' phosphoryl end back into the proximity of the 3' hydroxyl group of the cleaved DNA; (6) religation of the phosphodiester bond.
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