title: Active site substitutions delineate distinct classes of eubacterial flap endonuclease.
abstract: FENs (flap endonucleases) play essential roles in DNA replication, pivotally in the resolution of Okazaki fragments. In eubacteria, DNA PolI (polymerase I) contains a flap processing domain, the N-terminal 5'-->3' exonuclease. We present evidence of paralogous FEN-encoding genes present in many eubacteria. Two distinct classes of these independent FEN-encoding genes exist with four groups of eubacteria, being identified based on the number and type of FEN gene encoded. The respective proteins possess distinct motifs hallmarking their differentiation. Crucially, based on primary sequence and predicted secondary structural motifs, we reveal key differences at their active sites. These results are supported by biochemical characterization of two family members--ExoIX (exonuclease IX) from Escherichia coli and SaFEN (Staphylococcus aureus FEN). These proteins displayed marked differences in their ability to process a range of branched and linear DNA structures. On bifurcated substrates, SaFEN exhibited similar substrate specificity to previously characterized FENs. In quantitative exonuclease assays, SaFEN maintained a comparable activity with that reported for PolI. However, ExoIX showed no observable enzymatic activity. A threaded model is presented for SaFEN, demonstrating the probable interaction of this newly identified class of FEN with divalent metal ions and a branched DNA substrate. The results from the present study provide an intriguing model for the cellular role of these FEN sub-classes and illustrate the evolutionary importance of processing aberrant DNA, which has led to their maintenance alongside DNA PolI in many eubacteria.
authors.affiliation: The University of Sheffield School of Medicine and Biomedical Sciences, Henry Wellcome Laboratories for Medical Research, Department of Infection and Immunity, Sheffield S102RX, UK.
authors.forename: Lee M
authors.initials: LM
authors.lastname: Allen
authors.forename: Michael R G
authors.initials: MR
authors.lastname: Hodskinson
authors.forename: Jon R
authors.initials: JR
authors.lastname: Sayers
mesh_terms: D000595:Amino Acid Sequence; D019943:Amino Acid Substitution / Q000502:physiology*; D001419:Bacteria / Q000201:enzymology* / Q000235:genetics; D020134:Catalytic Domain / Q000235:genetics* / Q000502:physiology; D003001:Cloning, Molecular; D004256:DNA Polymerase I / Q000737:chemistry / Q000235:genetics; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D005090:Exodeoxyribonucleases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D045585:Flap Endonucleases / Q000737:chemistry / Q000145:classification* / Q000235:genetics; D008954:Models, Biological; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010727:Phosphoric Diester Hydrolases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D010802:Phylogeny; D017385:Sequence Homology; D013211:Staphylococcus aureus / Q000201:enzymology / Q000235:genetics; D013329:Structure-Activity Relationship; D013379:Substrate Specificity
chemical_list: D004256:DNA Polymerase I; D005090:Exodeoxyribonucleases; D045585:Flap Endonucleases; C113094:exonuclease IX, E coli; D010727:Phosphoric Diester Hydrolases