CHARACTERIZATION AND SUBCELLULAR-LOCALIZATION OF RIBONUCLEASE-H ACTIVITIES FROM XENOPUS-LAEVIS OOCYTES

Ribonuclease H activities present in fully grown Xenopus oocytes were investigated by using either liquid assays or renaturation gel assays. Whereas the test in solution detected an apparently unique class I ri bonuclease H activity, the activity gels did not detect this enzyme bu t another one with the molecular weight expected for a class II ribonu clease H. The ribonuclease HI was found to be primarily concentrated i n the germinal vesicle, but around 5% of this activity was detected in the cytoplasm and may correspond to the activity involved in antisens e oligonucleotide-mediated destruction of messenger RNAs. The concentr ation of this class I ribonuclease H in oocytes is similar to that in somatic cells. The class II ribonuclease H remained undetectable by th e test in solution because its activity was cryptic. On activity gel, a polypeptide with the apparent molecular mass of 32 kDa, expected for a ribonuclease HII, was found to be concentrated in mitochondria alth ough no RNase H activity could be detected by using the liquid assay. Based on sedimentation studies, we hypothesize that the apparent absen ce of RNase H activity in solution could be the result of the associat ion of this 32-kDa polypeptide with other polypeptides, or possibly nu cleic acids, to form a multimer of, until now, unknown function.