Integron integrases possess a unique additional domain necessary for activity

Integrons are genetic elements capable of integrating genes by a site-speci fic recombination system catalyzed by an integrase. Integron integrases are members of the tyrosine recombinase family and possess the four invariant residues (RHRY) and conserved motifs (boxes I and H and patches I, II, and III). An alignment of integron integrases compared to other tyrosine recomb inases shows an additional group of residues around the patch III motif. We have analyzed the DNA binding and recombination properties of class I inte gron integrase (IntI1) variants carrying mutations at residues that are wel l conserved among all tyrosine recombinases and at some residues from the a dditional motif that are conserved among the integron integrases. The well- conserved residues studied were H277 from the conserved tetrad RHRY (about 90% conserved), E121 found in the patch I motif (about 80% conserved in pro karyotic recombinases), K171 from the patch II motif (near 100% conserved), W229 and F233 from the patch III motif, and G302 of box H (about 80% conse rved in prokaryotic recombinases). Additional IntI1 mutated residues were K 219 and a deletion of the sequence ALER215. We observed that E121, K171, an d G302 play a role in the recombination activity but can be mutated without disturbing binding to DNA. W229, F233, and the conserved histidine (H277) may be implicated in protein folding or DNA binding. Some of the extra resi dues of IntI1 seem to play a role in DNA binding (K219) while others are im plicated in the recombination activity (ALER215 deletion).