Priming of DNA replication in eukaryotes is carried out by a heterotetramer
ic complex known as pol alpha -primase. The structure of the Pyrococcus fur
iosus RNA polymerase subunit of this complex shows that priming is carried
out by unrelated proteins in archaea/eukarya and eubacteria, provides insig
hts into the mechanism of action of a novel RNA polymerase, and raises new
questions regarding the ancestry of priming and replication in the three do
mains of life.