title: Is modification sufficient to protect a bacterial chromosome from a resident restriction endonuclease?
abstract: It has been generally accepted that DNA modification protects the chromosome of a bacterium encoding a restriction and modification system. But, when target sequences within the chromosome of one such bacterium (Escherichia coli K-12) are unmodified, the cell does not destroy its own DNA; instead, ClpXP inactivates the nuclease, and restriction is said to be alleviated. Thus, the resident chromosome is recognized as 'self' rather than 'foreign' even in the absence of modification. We now provide evidence that restriction alleviation may be a characteristic of Type I restriction-modification systems, and that it can be achieved by different mechanisms. Our experiments support disassembly of active endonuclease complexes as a potential mechanism. We identify amino acid substitutions in a restriction endonuclease, which impair restriction alleviation in response to treatment with a mutagen, and demonstrate that restriction alleviation serves to protect the chromosome even in the absence of mutagenic treatment. In the absence of efficient restriction alleviation, a Type I restriction enzyme cleaves host DNA and, under these conditions, homologous recombination maintains the integrity of the bacterial chromosome.
authors.affiliation: Institute of Cell and Molecular Biology, Darwin Building, University of Edinburgh, King's Buildings, Edinburgh EH9 3JR, UK.
authors.forename: Svetlana
authors.initials: S
authors.lastname: Makovets
authors.forename: Lynn M
authors.initials: LM
authors.lastname: Powell
authors.forename: Annette J B
authors.initials: AJ
authors.lastname: Titheradge
authors.forename: Garry W
authors.initials: GW
authors.lastname: Blakely
authors.forename: Noreen E
authors.initials: NE
authors.lastname: Murray
mesh_terms: D000251:Adenosine Triphosphatases / Q000235:genetics / Q000378:metabolism; D002876:Chromosomes, Bacterial / Q000235:genetics*; D004262:DNA Restriction Enzymes / Q000235:genetics / Q000378:metabolism*; D004269:DNA, Bacterial / Q000235:genetics / Q000378:metabolism; D015253:Deoxyribonucleases, Type I Site-Specific / Q000235:genetics / Q000378:metabolism*; D049071:Endopeptidase Clp; D004926:Escherichia coli / Q000145:classification / Q000201:enzymology* / Q000235:genetics*; D029968:Escherichia coli Proteins; D005838:Genotype; D012697:Serine Endopeptidases / Q000235:genetics / Q000378:metabolism
chemical_list: D004269:DNA, Bacterial; D029968:Escherichia coli Proteins; D004262:DNA Restriction Enzymes; D015253:Deoxyribonucleases, Type I Site-Specific; D012697:Serine Endopeptidases; C483573:ClpXP protease, E coli; D049071:Endopeptidase Clp; D000251:Adenosine Triphosphatases