You are studying the characteristics of DNA isolated from a mutant strain of E coli. Your data show that it can be digested into numerous smaller fragments with a restriction endonuclease that is known to cleave at a GGATCC motif. In comparison, DNA from the wild-type parental strain is resistant to digestion with this restriction endonuclease. The mutation that causes this type of phenotype is most likely to occur in the gene encoding which of the following activities?

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The mutation likely affects the methylase gene, which normally methylates the GGATCC motif in the wild-type strain, protecting it from cleavage by the restriction endonuclease. In the mutant strain, the lack of methylation allows the endonuclease to cleave the DNA.  Restriction endonucleases are enzymes that will recognize, bind to, and hydrolyze specific nucleic acid sequences in double-stranded DNA. The term restriction endonuclease was given to this class of bacterially derived enzymes, since they were identified as being involved in restricting the growth of certain bacteriophages. Bacteria are capable of modifying specific sequences within their genomes by methylation, which prevents their own DNA from being recognized by the restriction enzymes encoded by their genomes.

You Are Studying the Characteristics of DNA Isolated from a Mutant