title: Analysis of DNA looping interactions by type II restriction enzymes that require two copies of their recognition sites.
abstract: Before cleaving DNA substrates with two recognition sites, the Cfr10I, NgoMIV, NaeI and SfiI restriction endonucleases bridge the two sites through 3D space, looping out the intervening DNA. To characterise their looping interactions, the enzymes were added to plasmids with two recognition sites interspersed with two res sites for site-specific recombination by Tn21 resolvase, in buffers that contained either EDTA or CaCl2 so as to preclude DNA cleavage by the endonuclease; the extent to which the res sites were sequestered into separate loops was evaluated from the degree of inhibition of resolvase. With Cfr10I, a looped complex was detected in the presence but not in the absence of Ca(2+); it had a lifetime of about 90 seconds. Neither NgoMIV nor NaeI gave looped complexes of sufficient stability to be detected by this method. In contrast, SfiI with Ca(2+) produced a looped complex that survived for more than seven hours, whereas its looping interaction in EDTA lasts for about four minutes. When resolvase was added to a SfiI binding reaction in EDTA followed immediately by CaCl2, the looped DNA was blocked from recombination while the unlooped DNA underwent recombination. By measuring the distribution between looped and unlooped DNA at various SfiI concentrations, and by fitting the data to a model for DNA binding by a tetrameric protein to two sites in cis, an equilibrium constant for the looping interaction was determined. The equilibrium constant was essentially independent of the length of DNA between the SfiI sites.
authors.affiliation: Department of Biochemistry School of Medical Sciences, University of Bristol, Bristol, BS8 1TD, UK.
authors.forename: S E
authors.initials: SE
authors.lastname: Milsom
authors.forename: S E
authors.initials: SE
authors.lastname: Halford
authors.forename: M L
authors.initials: ML
authors.lastname: Embleton
authors.forename: M D
authors.initials: MD
authors.lastname: Szczelkun
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism; D001665:Binding Sites; D002118:Calcium / Q000378:metabolism; D004247:DNA / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism*; D004268:DNA-Binding Proteins / Q000378:metabolism; D015252:Deoxyribonucleases, Type II Site-Specific / Q000378:metabolism*; D004720:Endonucleases / Q000378:metabolism; D018628:Gene Dosage; D007700:Kinetics; D009690:Nucleic Acid Conformation; D010957:Plasmids / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D011995:Recombination, Genetic / Q000235:genetics; D012091:Repetitive Sequences, Nucleic Acid / Q000235:genetics; D013379:Substrate Specificity; D013696:Temperature; D013816:Thermodynamics; D045564:Transposon Resolvases
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D004247:DNA; C473490:Tn21 resolvase; D045564:Transposon Resolvases; D004720:Endonucleases; C066244:endodeoxyribonuclease Cfr10I; C060630:endodeoxyribonuclease NaeI; C041749:endodeoxyribonuclease SfiI; D015252:Deoxyribonucleases, Type II Site-Specific; C413605:NgoMIV endonuclease; D002118:Calcium