Structure of the RNA polymerase domain of E-coli primase

All cellular organisms use specialized RNA polymerases called "primases" to synthesize RNA primers for the initiation of DNA replication. The high-res olution crystal structure of a primase, comprising the catalytic core of th e Escherichia coli DnaG protein, was determined. The core structure contain s an active-site architecture that is unrelated to other DNA or RNA polymer ase palm folds, but is instead related to the "toprim" fold. On the basis o f the structure, it is Likely that DnaG binds nucleic acid in a groove clus tered with invariant residues and that DnaG is positioned within the replis ome to accept single-stranded DNA directly from the replicative helicase.