MECHANISMS OF NONHOMOLOGOUS DNA END-JOINING IN FROGS, MICE AND MEN

DNA end-joining, a process related to illegitimate recombination and c apable of rejoining unrelated pairs of DNA ends in the absence of sequ ence homology, is considered the major pathway of double-strand break (DSB) repair in mammalian cells. Whole cell and nuclear extracts from three human and one mouse cell line were investigated for their capaci ties to promote nonhomologous DNA end-joining and their relative activ ities of DNA-PK, a mammalian DNA end-binding protein complex implicate d in DSB-repair. The levels of DNA end-joining and the spectra of junc tions of the human systems were identical with the ones of a previousl y described cell-free joining system derived from Xenopus laevis eggs. Due to the presence of potent 3'-5'-exonuclease activities the mouse system displayed decreased levels of DNA end-joining and larger fracti ons of junctions containing deletions but otherwise the basic mechanis ms of junction formation appeared to be identical with the Xenopus sys tem. DNA-PK activity was found to be equally low in the Xenopus and th e mouse system but 4- to 6-fold increased in the human systems. Our re sults suggest that the mechanisms of DNA end-joining may be modulated by the level of exonuclease activities and/or DNA end-protecting facto rs but are otherwise highly conserved in vertebrate cells.