All restriction enzymes examined are phosphodiesterases generating 3'-OH and 5'-P ends, but one restriction enzyme (restriction glycosylase) excises unmethylated bases from its recognition sequence. Whether its restriction activity involves endonucleolytic cleavage remains unclear. One report on this enzyme, R. PabI from a hyperthermophile, ascribed the breakage to high temperature while another showed its weak AP lyase activity generates atypical ends. Here, we addressed this issue in mesophiles. We purified R. PabI homologs from Campylobacter coli (R. CcoLI) and Helicobacter py-lori (R. HpyAXII) and demonstrated their DNA cleavage, DNA glycosylase and AP lyase activities in vitro at 37. C. The AP lyase activity is more coupled with glycosylase activity in R. CcoLI than in R. PabI. R. CcoLI/R. PabI expression caused restriction of incoming bacteriophage/plasmid DNA and endogenous chromosomal DNA within Escherichia coli at 37 degrees C. The R. PabI-mediated restriction was promoted by AP endonuclease action in vivo or in vitro. These results reveal the role of endonucleolytic DNA cleavage in restriction and yet point to diversity among the endonucleases. The cleaved ends are difficult to repair in vivo, which may indicate their biologi-cal significance. These results support generalization of the concept of restriction-modification system to the concept of self-recognizing epigenetic system, which combines any epigenetic labeling and any DNA damaging.