DNA polymerase III is a complex, multichain enzyme responsible for most of the replicative synthesis in bacteria. This DNA polymerase also exhibits 3' to 5' exonuclease activity (By similarity). The exact function of the theta subunit is unknown. a 2'-deoxyribonucleoside 5'-triphosphate + DNA(n) = diphosphate + DNA(n+1) The DNA polymerase holoenzyme is a complex that contains 10 different types of subunits. These subunits are organized into 3 functionally essential subassemblies: the pol III core, the beta sliding clamp processivity factor and the clamp-loading complex. The pol III core (subunits alpha,epsilon and theta) contains the polymerase and the 3'-5' exonuclease proofreading activities. The polymerase is tethered to the template via the sliding clamp processivity factor. The clamp-loading complex assembles the beta processivity factor onto the primer template and plays a central role in the organization and communication at the replication fork. This complex contains delta, delta', psi and chi, and copies of either or both of two different DnaX proteins, gamma and tau. The composition of the holoenzyme is, therefore: (alpha,epsilon,theta)[2]-(gamma/tau)[3]-delta,delta', psi,chi-beta[4] (By similarity).