Isolation of a Mutant of Escherichia coli Defective in Cytosine-Specific Deoxyribonucleic Acid Methylase Activity and in Partial Protection of Bacteriophage λ Against Restriction by Cells Containing the N-3 Drug-Resistance Factor

摘要

A mutant (designated mec⁻) of Escherichia coli F⁺ 100 endo I⁻su⁺ rK⁻mK⁺ has been isolated which is defective in cytosine-specific deoxyribonucleic acid (DNA) methylase activity. The DNA of this mutant, as well as the DNA of phages λ and fd propagated in it, is virtually devoid of 5-methyl-cytosine (MeC); in contrast, the mutation has no significant effect on the level of N⁶-methyladenine in DNA. Phage λ grown on the mec⁻ mutant is more strongly restricted by N-3-containing cells than is λ grown on the mec⁺ parent. These results suggest that methylation of certain cytosine residues by the E. coli K-12 enzyme partially protects λ DNA from either the N-3 restriction nuclease or against secondary degradation subsequent to N-3-specific degradation. Analysis of the MeC level in viral and cellular DNA obtained from mec⁺, mec⁺ (mN3⁺), and mec⁻ (mN3⁺) strains has led to the conclusion that the R-factor controlled DNA-cytosine methylase may be capable of methylating a sequence(s) which is a substrate for the K-12 enzyme.