Recognition of (dG)n.(dC)n sequences by endonuclease G. Characterization of the calf thymus nuclease.

We report the purification of endonuclease G (Ruiz-Carrillo, A., and Renaud, J. (1987) EMBO J. 6, 401-407) from calf thymus nuclei and whole tissue. The enzyme has been enriched 29,000-fold, and the activity was unambiguously identified with a 26-kDa protein after renaturation following sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The native nuclease behaves as a 50-kDa species by gel filtration, suggesting that it is composed of two subunits, presumably identical. In terms of absolute amounts, endonuclease G (endo G) is a nuclear enzyme although it was also detected in purified mitochondria. Endo G is highly specific for (dG)n.(dC)n tracts in DNA, nicking either strand of relaxed substrates with similar kinetics. The sensitivity of the homopolymer tracts is proportional to their length (from n = 8 to 29), insofar as the flanking sequences are constant. However, the overall rate of cleavage is influenced by the composition of the flanking DNA. Minor cleavage sites contain shorter (dG)n.(dC)n clusters (n = 3-7). Endo G efficiently cleaves (dC)n but not (dG)n runs in single-stranded DNA, suggesting that it may recognize an asymmetric strand conformation of the homopolymer tracts. Endo G does not recognize other homo(co)-polymer sequences or cruciform structures in DNA.