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  • Title: Specificity from the synapsis of DNA elements by the Sfi I endonuclease.
    Author: Embleton ML, Williams SA, Watson MA, Halford SE.
    Journal: J Mol Biol; 1999 Jun 18; 289(4):785-97. PubMed ID: 10369761.
    Abstract:
    The synapsis of DNA sites is a prerequisite for the reactions of many proteins that act at specific DNA sequences. The requirement for synapsis was investigated by analysing the reactions of Sfi I, a tetrameric restriction enzyme that cleaves DNA only after interacting with two recognition sites. In the presence of Mg2+, oligonucleotide duplexes with the cognate recognition sequence were cleaved rapidly, with cooperative kinetics, while non-cognate duplexes were not cleaved. In the absence of Mg2+, the primary complex formed by Sfi I with cognate DNA contained two duplexes synapsed by the tetramer: a secondary complex containing one duplex was seen only at elevated Sfi I concentrations. In contrast, the principal complex with non-cognate DNA contained one duplex bound to Sfi I. Pairs of non-cognate duplexes, or one cognate and one non-cognate duplex, generally failed to form synaptic complexes. On adding Mg2+to complexes with cognate DNA, cleavage occurred much more rapidly in the synaptic complex than in the secondary complex. DNA synapsis thus acts to enhance the specificity of Sfi I for its recognition sequence, by demanding two cognate sites for a catalytically active complex and by excluding non-cognate sites from the synaptic complex.
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