title: Rapid evolution of the DNA-binding site in LAGLIDADG homing endonucleases.
abstract: Sequence analysis of chloroplast and mitochondrial large subunit rRNA genes from over 75 green algae disclosed 28 new group I intron-encoded proteins carrying a single LAGLIDADG motif. These putative homing endonucleases form four subfamilies of homologous enzymes, with the members of each subfamily being encoded by introns sharing the same insertion site. We showed that four divergent endonucleases from the I-CreI subfamily cleave the same DNA substrates. Mapping of the 66 amino acids that are conserved among the members of this subfamily on the 3-dimensional structure of I-CreI bound to its recognition sequence revealed that these residues participate in protein folding, homodimerization, DNA recognition and catalysis. Surprisingly, only seven of the 21 I-CreI amino acids interacting with DNA are conserved, suggesting that I-CreI and its homologs use different subsets of residues to recognize the same DNA sequence. Our sequence comparison of all 45 single-LAGLIDADG proteins identified so far suggests that these proteins share related structures and that there is a weak pressure in each subfamily to maintain identical protein-DNA contacts. The high sequence variability we observed in the DNA-binding site of homologous LAGLIDADG endonucleases provides insight into how these proteins evolve new DNA specificity.
authors.affiliation: Centre de Recherche sur la Fonction, la Structure et l'Ingénierie des Protéines, Pavillon Charles-Eugène Marchand, Université Laval, Québec, Québec G1K 7P4, Canada.
authors.forename: P
authors.initials: P
authors.lastname: Lucas
authors.forename: C
authors.initials: C
authors.lastname: Otis
authors.forename: J P
authors.initials: JP
authors.lastname: Mercier
authors.forename: M
authors.initials: M
authors.lastname: Turmel
authors.forename: C
authors.initials: C
authors.lastname: Lemieux
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D001665:Binding Sites; D000460:Chlorophyta / Q000201:enzymology* / Q000235:genetics; D003001:Cloning, Molecular; D017124:Conserved Sequence / Q000235:genetics; D018360:Crystallography, X-Ray; D004262:DNA Restriction Enzymes / Q000737:chemistry; D004268:DNA-Binding Proteins / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D004720:Endonucleases / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D019143:Evolution, Molecular; D020459:Genes, rRNA / Q000235:genetics; D007438:Introns / Q000235:genetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity
chemical_list: D004268:DNA-Binding Proteins; D004720:Endonucleases; D004262:DNA Restriction Enzymes; C048901:endodeoxyribonuclease CreI