A deoxyribonucleic acid unwinding protein isolated from regenerating rat liver. Physical and functional properties.

A DNA-unwinding protein has been purified from regenerating rat liver cytosol to apparent homogeneity. The protein is present in about 10(6) copies per cell. It is a tetramer, composed of 25,000-dalton subunits which does not exhibit enzymatic activity for ATPase, DNA polymerase, or DNase. The protein is able to unwind the double helix of poly[d(A-T)], depressing the melting point of this synthetic polymer by about 40 degrees. It also binds to supercoiled SV40 DNA, probably by melting A-T-rich regions in the genome. The fully saturated complex of protein and SV40 DNA sediments at 30 S. Homologous DNA polymerases-alpha and -beta are stimulated by the protein at a different level depending on the templates used. This result argues in favor of the intervention of the unwinding protein in replication processes.