Base-excision restriction enzymes: expanding the world of epigenetic immune systems.

The restriction enzymes examined so far are phosphodiesterases, which cleave DNA strands by hydrolysing phosphodiester bonds. Based on the mobility of restriction-modification systems, recent studies have identified a family of restriction enzymes that excise a base in their recognition sequence to generate an abasic (AP) site unless the base is properly methylated. These restriction glycosylases also show intrinsic but uncoupled AP lyase activity at the AP site, generating an atypical strand break. Action of an AP endonuclease at the AP site may generate another atypical break, rejoining/repairing of which is difficult. This PabI family of restriction enzymes contain a novel fold (HALFPIPE) and show unusual properties, such as non-requirement of divalent cations for cleavage. These enzymes are present in Helicobacteraceae/Campylobacteraceae and in few hyperthermophilic archaeal species. In Helicobacter genomes, their recognition sites are strongly avoided, and the encoding genes are often inactivated by mutations or replacement, indicating that their expression is toxic for the cells. The discovery of restriction glycosylases generalizes the concept of restriction-modification systems to epigenetic immune systems, which may use any mode of damage to DNA that are considered 'non-self' based on epigenetic modifications. This concept will add to our understanding of immunity and epigenetics.