Yeast DNA primase is encoded by a 59-kilodalton polypeptide: purification and immunochemical characterization

The DNA primase from the yeast Saccharomyces cerevisiae has been purified 9200-fold to homogeneity. The yeast DNA primase is a monomeric protein of molecular weight 59,000, and under conditions described in this report, it is stable at 4 or -80 degrees C. The primase does not bind to DEAE-cellulose, is not inhibited by a high concentration of alpha-amanitin (4 mg/mL), and is capable of synthesizing small (up to 15 nucleotides in length) ribo or ribo-deoxy mixed initiator RNA primers. The primer synthesis is stimulated by ATP; however, other ribonucleotides could be replaced by deoxynucleotides without any measurable effect on the overall DNA synthesis. Thus, the purified primase is distinct from the RNA polymerases of S. cerevisiae. Immunoblot analysis of the polypeptides in a crude cell extract using a mouse polyclonal antibody prepared against the highly purified primase indicates that the 59-kilodalton polypeptide is the native form and not a degraded form of a larger polypeptide; however, primase is degraded rapidly to smaller polypeptides by yeast proteases especially in the absence of protease inhibitors.