DNA primase activity from human lymphocytes. Synthesis of oligoribonucleotides that prime DNA synthesis.

A fraction has been prepared from extracts of a human lymphoblastoid cell line that has properties of a mammalian DNA primase and also contains a DNA polymerase activity with unusual properties. With a variety of synthetic single-stranded DNA templates using rNTPs alone, the products consist of oligoribonucleotides of a restricted size range, primarily 7 to 9 nucleotides in length. Poly(dIT) is the most active template found thus far. The activity appears to have "relaxed" substrate/template complementarity requirements similar to those described previously for mammalian primase; poly(dIT) template with rATP alone results in synthesis of oligo(rA) of the same size as oligo(rAC) made when both rATP and rCTP are present. When dNTPs are added to the reaction, DNA is synthesized by extension of the oligoribonucleotide, which acts as primer. The DNA product appears in relatively discrete sizes that differ by approximately 8 nucleotides, with a large proportion of the product around 24 and 32 nucleotides. In addition to the relatively discrete size of its product, the DNA polymerase activity that utilizes the endogenously synthesized oligoribonucleotide primer on poly(dIT) template differs from polymerase alpha in its resistance to aphidicolin and low Km for dNTP.