Endonucleolytic cleavage of pBR 322 DNA by a Yoshida ascites cell DNA binding protein

A 34 000 dalton DNA-binding protein has been purified from Yoshida ascites tumor cells to electrophoretic homogeneity. Nitrocellulose filter binding assays showed preferential binding of this protein to single-stranded DNA. This protein cleaved pBR 322 DNA and the cleavage products showed the presence of a major band corresponding to the circular form of the DNA when analysed in agarose gels under non-denaturing conditions. Under denaturing conditions, two major DNA bands, one corresponding to the single-stranded circular form and the other to linear form were obtained. These results indicate that the 34 000 D protein possesses an endonucleolytic activity which cleaves pBR 322 DNA by the introduction of a single cut in one of the DNA strands.