Partially deficient methylation of cytosine in DNA at CC TAGG sites stimulates genetic recombination of bacteriophage lambda

Abstract

Lambda bacteriophages grown on arl mutants of Escherichia coil (“Arl −” phages) display intermediate levels of cytosine methylation: less 5-methylcytosine (m 5C) than phages grown on wild-type bacteria (“Arl +” phages) but more than phages grown on dcm mutants, and thus lacking the methylated sequences (Cm 5C T AGG) characteristic of E. coil K-12 bacteria (“Dcm −” phages). “Arl −” phages are one twelfth as resistant to Eco RII restriction (recognition site CC T AGG) as “Arl +” phages, but 40-fold more resistant than “Dcm −” phages. Chromatographic analyses show the 5-methylcytosine content of “Arl −” DNA to be one third that of “Arl +” DNA. Altered cytosine methylation frequency correlates with two previously described properties of “Arl −” phages, increased genetic recombination and unusual sensitivity of phage DNA to endonuclease S1, which are absent in phages grown on dcm or dcm arl bacteria. Methylated/unmethylated heteroduplex DNA prepared in vitro (one strand from Eco RII-modified phages/one from “Dcm −” phages) is highly recombinogenic but not S1-sensitive. We hypothesize that hemimethylated CC T AGG sites in “Arl −” DNA are necessary and sufficient for enhanced recombination, and necessary but not sufficient for S1 sensitivity.