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Novel mutations of Escherichia coli that produce recombinogenic lesions in DNA: V. Recombinogenic plasmids from arl mutants of Escherichia coli are unusually sensitive to nuclease S1 and partially deficient in cytosine methylation at C-C-(A/T)-G-G sequences
Authors:Brent E Korba  John B Hays
Institution:Department of Chemistry University of Maryland Baltimore County Catonsville, Md 21228, U.S.A.
Abstract:Two novel phenotypes previously associated with arl mutations of Escherichia coli, increased frequencies of genetic recombination and unusual sensitivity of DNA to the single-strand-specific nuclease S1, have been defined most completely by the properties of λ bacteriophages grown on arl bacteria (Arl? phages). We now find that plasmids maintained in arl mutants (Arl? plasmids) exhibit elevated recombination frequencies, unusual sensitivity to nuclease S1 (in a limited number of regions) and a new Arl phenotype, partially deficient methylation of the inner cytosine at C-C-(A/T)-G-G sequences.A variety of Arl? plasmids (all pBR322 derivatives) show elevated recombination (4 to 10-fold) by three different assays (frequencies of homomultimers and of heteromultimers, efficiency of intramolecular recombination). Plasmids from arl bacteria (after conversion to linear form) are nicked by nuclease S1 about 0.7 times per duplex; Arl+ plasmids are nuclease S1-resistant. Restriction endonuclease EcoRII (recognition sequence, C-C-(A/T)-G-G) cuts Arl? plasmid DNA more readily than Arl+ DNA, but Arl? plasmids are still more EcoRII-resistant than Dcm? plasmids (from E. coli dcm mutants, which lack the chromosomal cytosine methylase; recognition sequence, also C-C-(A/T)-G-G). By chromatographic analyses, Arl? plasmid DNA contains less 5-methylcytosine than Arl+ (0.07% versus 0.15%). although the 6-methyladenine content is the same (0.5mol%).
Keywords:kb  meC  5-methylcytosine  meA  6-methyladenine
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