J. Biol. Chem., Vol. 283, Issue 17, 99913, April 25, 2008
The Polymerase Exchange
During DNA replication in Escherichia coli, the DNA polymerase III holoenzyme will stall upon encountering a lesion. At this point, special translesion synthesis (TLS) polymerases like pol IV take over and bypass the bad site, allowing normal replication to continue. It remains unclear how TLS polymerases gain access to the primer site whereas stalled pol III sits there, although a "tool belt" model has been proposed where both enzymes are attached to the polymerase β subunit (the sliding clamp) and switch on and off as needed. In this Paper of the Week, Asako Furukohri and colleagues propose an alternative where pol IV displaces pol III. Using an assay measuring the DNA synthesis burst of primed pol III following the addition of dTTP, they found that adding pol IV before or after the nucleotide inhibited the synthesis burst. During this slow-down, the pol III* complex (two core units and the DnaX protein) no longer fractionated with the template DNA, instead being replaced by pol IV. The researchers propose a dynamic exchange model, where a stalled pol III complex enables pol IV to replace the stuck enzyme, and following lesion bypass, free pol III exchanges back in through mass action.
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Three proposed models for pol III*-pol IV dynamic exchange.
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FOOTNOTES
See referenced article, J. Biol. Chem. 2008, 283, 11260-11269 
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Copyright © 2008 by the American Society for Biochemistry and Molecular Biology.
