Primase initiates Okazaki pieces during polyoma DNA synthesis

Abstract

SHORT fragments (Okazaki pieces) are formed as intermediates during elongation of nascent DNA strands1. In isolated nuclei from polyoma-virus infected 3T6 cells Okazaki pieces are initiated by a short RNA primer (initiator RNA, iRNA) with the approximate size of a decanucleotide2. The nature of the polymerase responsible for primer synthesis is not known, but high concentrations of α-amanitin, a drug which inhibits both RNA polymerase II (ref. 3) and III (ref. 4), have no effect on primer formation (unpublished results). We now present evidence that iRNA is not synthesised by any of the known RNA polymerases, but that, instead, a mammalian enzyme is involved which in its action corresponds to the dnaG gene product of E. coli. The bacterial enzyme was named primase5. It participates in the initiation of DNA strands of some single-stranded bacteriophages and has been suggested to be involved in the synthesis of bacterial Okazaki pieces6,7. One of the distinguishing features of the E. coli enzyme is its ability to use both ribonucleotides and deoxyribonucleotides for primer synthesis5,8.