In E. coli, the adenine in the sequence GATC is methylated by the Dam enzyme. In the same cells, a restriction endonuclease recognizes and cleaves dsDNA with the sequence GATC on either strand. Why does E. coli have these two enzymes?
A) The enzymes cut the E. coli genome into pieces that bind to viral particles and inhibit viral replication.
B) The enzymes increase the rate of mutation and genome rearrangement, thus increasing the likelihood that E. coli cells will mutate and become resistant to viral infection.
C) The enzymes encourage lysogeny because the cleavage sites are recognized by viral integrases.
D) The enzymes protect E. coli from infection by preferentially degrading viral or other exogenous DNA that is not methylated.

Question

Quizplus · Accepted Answer

The presence of the restriction endonuclease that recognizes and cleaves unmethylated GATC sequences suggests that its function is to protect E. coli from infection by preferentially degrading viral or other exogenous DNA that is not methylated. The Dam methylase protects the E. coli genome from being cleaved by the restriction endonuclease by methylating the adenine in the GATC sequence, thus preventing the enzyme from recognizing it. This system is known as the restriction-modification system and is a common mechanism of bacterial defense against foreign DNA.

In E Coli, the Adenine in the Sequence GATC Is Methylated