title: DNA cleavage and methylation specificity of the single polypeptide restriction-modification enzyme LlaGI.
abstract: LlaGI is a single polypeptide restriction-modification enzyme encoded on the naturally-occurring plasmid pEW104 isolated from Lactococcus lactis ssp. cremoris W10. Bioinformatics analysis suggests that the enzyme contains domains characteristic of an mrr endonuclease, a superfamily 2 DNA helicase and a gamma-family adenine methyltransferase. LlaGI was expressed and purified from a recombinant clone and its properties characterised. An asymmetric recognition sequence was identified, 5'-CTnGAyG-3' (where n is A, G, C or T and y is C or T). Methylation of the recognition site occurred on only one strand (the non-degenerate dA residue of 5'-CrTCnAG-3' being methylated at the N6 position). Double strand DNA breaks at distant, random sites were only observed when two head-to-head oriented, unmethylated copies of the site were present; single sites or pairs in tail-to-tail or head-to-tail repeat only supported a DNA nicking activity. dsDNA nuclease activity was dependent upon the presence of ATP or dATP. Our results are consistent with a directional long-range communication mechanism that is necessitated by the partial site methylation. In the accompanying manuscript [Smith et al. (2009) The single polypeptide restriction-modification enzyme LlaGI is a self-contained molecular motor that translocates DNA loops], we demonstrate that this communication is via 1-dimensional DNA loop translocation. On the basis of this data and that in the third accompanying manuscript [Smith et al. (2009) An Mrr-family nuclease motif in the single polypeptide restriction-modification enzyme LlaGI], we propose that LlaGI is the prototype of a new sub-classification of Restriction-Modification enzymes, named Type I SP (for Single Polypeptide).
authors.affiliation: DNA-Protein Interactions Unit, Department of Biochemistry, School of Medical Sciences, University of Bristol, Bristol, BS8 1TD, UK.
authors.forename: Rachel M
authors.initials: RM
authors.lastname: Smith
authors.forename: Fiona M
authors.initials: FM
authors.lastname: Diffin
authors.forename: Nigel J
authors.initials: NJ
authors.lastname: Savery
authors.forename: Jytte
authors.initials: J
authors.lastname: Josephsen
authors.forename: Mark D
authors.initials: MD
authors.lastname: Szczelkun
mesh_terms: D053837:DNA Cleavage; D019175:DNA Methylation; D015280:DNA Restriction-Modification Enzymes / Q000145:classification / Q000378:metabolism*; D007700:Kinetics; D009711:Nucleotides / Q000378:metabolism; D013379:Substrate Specificity
chemical_list: D015280:DNA Restriction-Modification Enzymes; C431608:LlaGI restriction-modification system; D009711:Nucleotides