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

Lambda bacteriophages grown on arl mutants of Escherichia coli ("Arl-" phages) display intermediate levels of cytosine methylation: less 5-methylcytosine (m5C) than phages grown on wild-type bacteria ("Arl+" phages) but more than phages grown on dcm mutants, and thus lacking the methylated sequences (Cm5CATGG) characteristic of E. coli K-12 bacteria ("Dcm-" phages). "Arl-" phages are one twelfth as resistant to Eco RII restriction (recognition site CCATGG) 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 CCATGG sites in "Arl-" DNA are necessary and sufficient for enhanced recombination, and necessary but not sufficient for S1 sensitivity.