Chromosoma (Berl.) 82, 583-593 (1981) CHROMOSOMA 9 Springer-Verlag 198l A DNA Binding Protein from Xenopus laevis Oocyte Mitoehondria Monique Barat and Bernard Mignotte Laboratoire de Biologie G6n6rale, Bfitiment 400, Universit~ Paris-Sud, 91405 Orsay, France Abstract. A DNA binding protein has been isolated, by affinity chromatogra- phy on DNA cellulose, from mitochondria and from purified mitDNA- protein complexes from oocytes ofXenopus laevis. This 12,500 daltons protein is polymeric in its native form and binds to DNA with a high efficiency. It exhibits an apparently preferential binding to the single-stranded fiber of the D loop structures. Introduction It is now beyond doubt that mitochondrial DNA (mitDNA) is not "naked" in the organelle but is associated with proteins (Kuroiwa, 1976; Pinon et al., 1978; Van Tuyle et al., 1979; Olszewka et al., 1980; Rickwood et al., submitted). MitDNA is thus condensed and acquires a chromatin-like condition as has been observed in Xenopus laevis (Pinon et al., 1978) and Paramecium aurelia (Olszewka et al., 1980). A basic protein has also been purified from yeast mito- chondria (Caron et al., 1979); it is able to introduce superhelical turns into circular relaxed SV40 DNA but there is no evidence that it binds to mitDNA either in vitro or in vivo. In our attempt to characterize further the Xenopus laevis mitDNA-protein complex we have found a protein which has a high affinity for DNA. In this report the purification of this protein is described, some of its properties charac- terized and strong evidence that it is associated with mitDNA in vivo are presented. Material and Methods Oocytes. Whole ovaries were taken from adult female Xenopus laevis and cut into pieces in Barth's medium as modified by Gurdon (1968). The animals were purchased from STACEL France. Mitochondrial Extract. All operations were carried out in the cold. The oocytes were homogenized in 0.25 M sucrose, 10 mM Tris-HC1 pH 7.5, 1 mM EDTA, 0,1% bovine serum albumin (BSA). 0009-5915/81/0082/0583/$02.20