Human placenta DNA primase: purification of enzyme and analysis of RNA primer synthesis.

The immunoaffinity purification of human placenta DNA primase devoid of DNA polymerase alpha activity is described. Primase consists of 52 and 59 kDa polypeptides. They form a single protein of 330 kDa under native conditions. The polypeptide structure of primase is believed to be (52 + 59)3. Primase synthesizes the oligoribonucleotides 2-9 monomers long and multimeric oligoribonucleotides of a modal length of about 10 monomers. The following model of RNA primer synthesis is proposed: 1) primase, being in free state or in complex with Pol alpha, forms a protein trimer or another structure that includes several primases; 2) primase synthesizes de novo only the oligonucleotides 2-10 monomers in length; 3) the newly synthesized oligonucleotides dissociate in solution or translocate to either Pol alpha or a neighbouring primase unit to be further elongated with the next 7-10 mononucleotides.