Restriction Enzyme and Bacterial Defense

Bacteria protect their own DNA from the potentially lethal effect of their restriction enzymes by previously modifying it, usually by an appropriate DNA methylase. This involves methylation of certain bases at a limited number of sequences so that the enzymes no longer recognize them as sites. DNA methylase binds at specific recognition sites and methylates certain adenines (A) or cytosines (C) within this sequence, preventing subsequent restriction endonuclease activity.

The function of a DNA methylase in bacteria is to methylate (add CH3 groups to) certain adenines (A) or cytosines (C) within specific recognition sequences, thus preventing subsequent restriction endonuclease activity and protecting the bacteria's own DNA from the potentially lethal effect of their restriction enzymes.

Restriction enzymes recognize foreign DNA by identifying specific recognition sequences, which are usually 4-6 nucleotides long, and making double-stranded cuts in the foreign DNA at these sequences.

The nature of the cleavage by restriction enzymes is important because the resulting ends determine the suitability of the fragments for subsequent procedures. All restriction enzymes cut their DNA substrates to produce a 5’-- phosphate and a 3’-hydroxyl terminal on each strand at each cut.

The smaller the number of nucleotides in the recognition site, the more often the restriction enzyme will tend to cut the DNA.