DNA primases

DNA primases are enzymes whose continual activity is required at the DNA re plication fork. They catalyze the synthesis of short RNA molecules used as primers for DNA polymerases. Primers are synthesized from ribonucleoside tr iphosphates and are four to fifteen nucleotides long. Most DNA primases can be divided into two classes. The first class contains bacterial and bacter iophage enzymes found associated with replicative DNA helicases. These prok aryotic primases contain three distinct domains: an amino terminal domain w ith a zinc ribbon motif involved in binding template DNA, a middle RNA poly merase domain, and a carboxyl-terminal region that either is itself a DNA h elicase or interacts with a DNA helicase. The second major primase class co mprises heterodimeric eukaryotic primases that form a complex with DNA poly merase alpha and its accessory B subunit. The small eukaryotic primase subu nit contains the active site for RNA synthesis, and its activity correlates with DNA replication during the cell cycle.