Deoxypyrimidine cluster mediates the priming by calf thymus DNA primase subunit.

Homogeneously purified DNA polymerase alpha subunit-free primase was used to analyze primer RNA synthesis. On a chemically synthesized 36 mer DNA template, a part of upstream region of human c-myc gene, the primer synthesis started from a doublet of deoxythymidine (TT) in the deoxypyrimidine-rich sequence. The primase in DNA polymerase alpha-primase complex synthesized 21-mer reaction product, while DNA polymerase alpha-free primase gave the similar products, 21- and 22-mer, indicating that the site recognition was carried out by primase itself and DNA polymerase alpha subunit has an auxiliary role on it. Product analysis using DNA fragments carrying base substitutions further revealed that the existence of deoxypyrimidine residues around the starting sites was important for priming frequencies. Competition analysis showed that the priming was strongly competed by poly(dC), and to a much lesser extent by poly(dA). Gel-shift analysis showed that the primase could bind to the DNA template, and this complex formation was also competed by poly(dC), but not by poly(dA). These results indicate that primase subunit interacts with the starting site by binding directly with deoxypyrimidine residues.