![]() Recently, the structure of the C-terminal domain (CTD) from the Listeria innocua prophage integrase (LI integrase) bound to its attP half-site was determined. Although the biochemical properties of some serine integrase systems have been studied in detail, a structural and mechanistic understanding of how these enzymes recognize their attachment sites and achieve such strong site selectivity and recombination directionality has been lacking. Indeed, several serine integrases have been shown to specifically and stably integrate target genes into mammalian cell lines. The high sequence-specificity and unidirectional nature of the integration reaction has made serine integrases attractive agents in gene therapy and genomic engineering applications. The integration reaction is effectively unidirectional the Int-bound attL and attR sites do not efficiently recombine to form attP and attB in the absence of a phage encoded recombination directionality factor (RDF). NTD: N-terminal catalytic domain (cyan) RD: recombinase domain (magenta) ZD: zinc ribbon domain (green) CC: coiled-coil motif (blue). vi) The unique arrangement of ZDs in the attL and attR sites allows the CC motifs to form intra-molecular interactions that prevent the reaction from running efficiently in the reverse direction. v) Base-pairing between the central dinucleotides promotes ligation of the DNA strands, resulting in formation of two new attachment sites, attL and attR. iv) The P′ and B′-linked subunits can exchange places by rotating 180° about a horizontal axis relative to the P and B-linked subunits. iii) The Int subunits cleave all four DNA strands at the central dinucleotide, forming 5′-phosphoserine linkages between integrase subunits and DNA half-sites (not illustrated) and generating 3′-dinucleotide overhangs. ii) Int- attP and Int- attB associate to form a synaptic complex that is stabilized by interactions between coiled-coil (CC) motifs. The Int- attP and Int- attB complexes are conformationally distinct due to different positioning of a zinc ribbon domain (ZD). ![]() i) Integrase (Int) dimers bind to specific sequences in the phage ( attP) and host ( attB) DNA. Recently proposed model of the integration reaction catalyzed by serine integrases. ![]()
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