title: A free-standing homing endonuclease targets an intron insertion site in the psbA gene of cyanophages.
abstract: Homing endonuclease genes are mobile elements that promote their duplication into cognate sites that lack the endonuclease gene [1, 2]. The homing endonuclease initiates this event through site-specific DNA cleavage. Copying of the endonuclease gene follows as a consequence of DNA repair. A genome containing a homing endonuclease gene is subject to self-cleavage. Protection is accomplished through DNA sequence polymorphisms, as is the case in intronless homing of free-standing endonuclease genes [3, 4], or by disruption of the recognition site by a group I intron (or intein) into which the endonuclease ORF is embedded. We describe here a novel free-standing homing endonuclease from cyanobacteriophage S-PM2, which is similar to the DNA resolvase of bacteriophage T4 and is encoded adjacent to an intron-containing psbA gene [5, 6]. The endonuclease makes a specific double-strand cut near the intron insertion site (IIS), its DNA recognition site spans the IIS, and it is unable to cleave intron-containing psbA genes. This interdependence of a free-standing endonuclease gene and a group I intron, which we denote "collaborative homing," has not been reported previously and gives support to a hypothesis of formation of composite mobile introns by independent convergence of an intron and an endonuclease gene on the same target sequence.
authors.affiliation: Department of Biological Sciences, University at Albany, SUNY, Albany, NY 12222, USA.
authors.forename: Qinglu
authors.initials: Q
authors.lastname: Zeng
authors.forename: Richard P
authors.initials: RP
authors.lastname: Bonocora
authors.forename: David A
authors.initials: DA
authors.lastname: Shub
mesh_terms: D001435:Bacteriophages / Q000201:enzymology* / Q000235:genetics; D004720:Endonucleases / Q000235:genetics*; D019143:Evolution, Molecular; D007438:Introns / Q000235:genetics*; D009841:Oligonucleotides / Q000235:genetics; D045332:Photosystem II Protein Complex / Q000235:genetics; D010957:Plasmids / Q000235:genetics; D048929:Prochlorococcus / Q000821:virology*; D014779:Virus Replication / Q000235:genetics*
chemical_list: D009841:Oligonucleotides; D045332:Photosystem II Protein Complex; C422369:photosystem II, psbA subunit; D004720:Endonucleases