Prokaryotes contain five different types of DNA polymerase. These are described below.
Polymerase I is a DNA repair enzyme from the family A polymerases that has a 5’ to 3’ and 3’ to 5’ activity. Pol I accounts for more than 95% of polymerase activity in E. coli, although cells that lack this polymerase have been found and its activity can be replaced by the other four types of polymerase. This DNA polymerase has a poor processivity rate, adding around 15 to 20 nucleotides per second. Pol I begins the process of DNA elongation at a point called the “origin of replication” and about 400 base pairs downstream of this point, Pol III takes over replication, which it performs at a much higher speed.
Polymerase II is a DNA repair enzyme with a 3’ to 5’ exonuclease activity. Pol II is a family B polymerase and provides support to Pol III. When DNA acquires damage in the form of short gaps, which block Pol III activity, Pol II helps to remedy this problem by restarting DNA synthesis downstream of these gaps.
This holoenzyme is the main polymerase in E.coli DNA replication and is one of the family C polymerases. Polymerase III is made up of the clamp-loading complex, the beta sliding clamp processivity factor and the Pol III core. The core comprises three subunits – the α subunit which is the polymerase activity hub, the δ subunit which is the exonucleolytic proofreader, and the θ subunit which may stabilize δ. The core and the beta sliding clamp are present in duplicate, to allow for processing of both the leading and lagging DNA strands.
This enzyme belongs to the Y family of DNA polymerases. Pol IV is an error-prone polymerase that has no 3’ to 5’ proofreading activity and is involved in mutagenesis or the altering of DNA to give rise to a mutation. The enzyme is expressed by a gene (dinB) that is switched on when polymerases stall at the replication fork. This interferes with the processivity of Pol III which acts as a checkpoint, stopping replication and allowing time for DNA to be repaired. Cells that lack dinB are at an increased risk of developing mutations caused by agents that damage DNA.
Pol V also belongs to the Y family of polymerases and allows DNA damage to be bypassed in order for replication to continue.
Reviewed by Sally Robertson, BSc