Recombinase, conserved site <p>Site-specific recombination plays an important role in DNA rearrangement in prokaryotic organisms. Two types of site-specific recombination are known to occur:</p><ol><li>Recombination between inverted repeats resulting in the reversal of a DNA segment.</li><li>Recombination between repeat sequences on two DNA molecules resulting in their cointegration, or between repeats on one DNA molecule resulting in the excision of a DNA fragment.</li></ol><p>Site-specific recombination is characterised by a strand exchange mechanism that requires no DNA synthesis or high energy cofactor; the phosphodiester bond energy is conserved in a phospho-protein linkage during strand cleavage and re-ligation.</p><p>Two unrelated families of recombinases are currently known [<cite idref="PUB00001148"/>]. The first, called the 'phage integrase' family, groups a number of bacterial, phage and yeast plasmid enzymes. The second [<cite idref="PUB00003786"/>], called the 'resolvase' family, groups enzymes which share the following structural characteristics: an N-terminal catalytic and dimerization domain that contains a conserved serine residue involved in the transient covalent attachment to DNA, and a C-terminal helix-turn-helix DNA-binding domain.</p><p>The resolvase family is currently known to include the following proteins:</p><ul><li>DNA invertase from <taxon tax_id="602">Salmonella typhimurium</taxon> (gene hin). Hin can invert a 900 bp DNA fragment adjacent to a gene for one of the flagellar antigens.</li><li>DNA invertase from <taxon tax_id="562">Escherichia coli</taxon> (gene pin).</li><li>DNA invertase from <taxon tax_id="10677">Bacteriophage Mu</taxon> (gene gin), P1 and P7 (gene cin).</li><li>Resolvases from transposons Tn3, Tn21, Tn501, Tn552, Tn917, Tn1546, Tn1721, Tn2501 and Tn1000 (known as gamma-delta resolvase).</li><li>Resolvase from <taxon tax_id="1502">Clostridium perfringens</taxon> plasmid pIP404.</li><li>Resolvase from E. coli plasmid R46.</li><li>Resolvase from E. coli plasmid RP4 (gene parA).</li><li>A putative recombinase from <taxon tax_id="1423">Bacillus subtilis</taxon> (gene cisA) [<cite idref="PUB00002098"/>] which plays an important role in sporulation by catalyzing the recombination of genes spoIIIC and spoIVCB to form polymerase sigma-K factor.</li><li>Uvp1, a protein from E. coli plasmid pR which cooperates with the mucAB genes in the DNA repair process and could be a resolvase [<cite idref="PUB00003732"/>].</li></ul><p>Generally, proteins from the resolvase family have 180 to 200 amino-acid residues, excepting cisA which is much larger (500 residues).</p>