Activation of a system for the joining of nonhomologous DNA ends during Xenopus egg maturation.

Mature Xenopus laevis eggs provide an elementary reaction system of illegitimate recombination which efficiently joins nonhomologous DNA ends (P. Pfeiffer and W. Vielmetter, Nucleic Acids Res. 16:907-924, 1988). Here we show that stage VI oocytes, known to express a system for homologous recombination (D. Carroll, Proc. Natl. Acad. Sci. USA 80:6902-6906, 1983), are completely devoid of this joining system. Nonhomologous DNA end-to-end joining, however, attains full activity only at an extremely late stage of egg maturation. Cycloheximide inhibition patterns indicate that nonhomologous joining activity is regulated at the G2 restriction point of the cell cycle. Implications of homologous and nonhomologous recombination activities during egg maturation are discussed.