Replicative DNA polymerases.
SUMMARY: Replicative DNA polymerases are essential for the replication of the genomes of all living organisms. On the basis of sequence similarities they can be classified into three types. Type A polymerases are homologous to bacterial polymerases I, Type B comprises archaebacterial DNA polymerases and eukaryotic DNA polymerase alpha, and the bacterial polymerase III class make up type C. Structures have been solved for several type A and B polymerases, which share a similar architecture. The structure of type C is not yet known. The catalytic mechanism of all three types involves two metal-ion-binding acidic residues in the active site. Replicative polymerases are constitutively expressed, but their activity is regulated through the cell cycle and in response to different growth conditions.