Trading places on DNA-a three-point switch underlies primer handoff from primase to the replicative DNA polymerase

This study reports a primase-to-polymerase switch in E. coli that closely l inks primase action with extension by DNA polymerase III holoenzyme. We fin d that primase tightly grips its RNA primer, protecting it from the action of other proteins. However, primase must be displaced before the beta slidi ng clamp can be assembled on the primed site. A single subunit of the holoe nzyme, chi, is dedicated to this primase displacement task. The displacemen t mechanism depends on a third protein, SSB. Primase requires contact to SS B for its grip on the primed site. The chi subunit also binds SSB, upon whi ch the primase-to-SSB contact is destabilized leading to dissociation of pr imase and assembly of beta onto the RNA primer. The conservation of this th ree-point switch, in which two proteins exchange places on DNA via mutually exclusive interaction with a third protein, is discussed.