title: I-Ssp6803I: the first homing endonuclease from the PD-(D/E)XK superfamily exhibits an unusual mode of DNA recognition.
abstract: BACKGROUND
  Restriction endonucleases (REases) and homing endonucleases (HEases) are biotechnologically important enzymes. Nearly all structurally characterized REases belong to the PD-(D/E)XK superfamily of nucleases, while most HEases belong to an unrelated LAGLIDADG superfamily. These two protein folds are typically associated with very different modes of protein-DNA recognition, consistent with the different mechanisms of action required to achieve high specificity. REases recognize short DNA sequences using multiple contacts per base pair, while HEases recognize very long sites using a few contacts per base pair, thereby allowing for partial degeneracy of the target sequence. Thus far, neither REases with the LAGLIDADG fold, nor HEases with the PD-(D/E)XK fold, have been found.
  
  
  RESULTS
  Using protein fold recognition, we have identified the first member of the PD-(D/E)XK superfamily among homing endonucleases, a cyanobacterial enzyme I-Ssp6803I. We present a model of the I-Ssp6803I-DNA complex based on the structure of Type II restriction endonuclease R.BglI and predict the active site and residues involved in specific DNA sequence recognition by I-Ssp6803I. Our finding reveals a new unexpected evolutionary link between HEases and REases and suggests how PD-(D/E)XK nucleases may develop a 'HEase-like' way of interacting with the extended DNA sequence. This in turn may be exploited to study the evolution of DNA sequence specificity and to engineer nucleases with new substrate specificities.
authors.affiliation: Laboratory of Bioinformatics and Protein Engineering, International Institute of Molecular and Cell Biology, Trojdena 4, 02-109 Warsaw, Poland.
authors.forename: Jerzy
authors.initials: J
authors.lastname: Orlowski
authors.forename: Michal
authors.initials: M
authors.lastname: Boniecki
authors.forename: Janusz M
authors.initials: JM
authors.lastname: Bujnicki
mesh_terms: D020816:Amino Acid Motifs; D000595:Amino Acid Sequence; D001665:Binding Sites; D004247:DNA / Q000737:chemistry / Q000378:metabolism*; D030562:Databases, Protein; D015252:Deoxyribonucleases, Type II Site-Specific / Q000737:chemistry* / Q000145:classification / Q000378:metabolism; D019143:Evolution, Molecular; D008958:Models, Molecular; D008969:Molecular Sequence Data; D017510:Protein Folding; D017433:Protein Structure, Secondary; D016415:Sequence Alignment; D013379:Substrate Specificity
chemical_list: D004247:DNA; D015252:Deoxyribonucleases, Type II Site-Specific; C018492:GCCNNNNNGGC-specific type II deoxyribonucleases