Replication complex associated enzymic activities

Polymerase I plays an essential role in the replication process in E. coli, but it is not responsible for the overall polymerization of the replicating strands. The enzyme that accomplishes this is a less abundant enzyme, polymerase III (pol III). (A DNA polymerase II has also been isolated from E. coli, but it probably plays no role in DNA synthesis.) Pol III catalyzes the same polymerization reaction as pot I but has certain distinguishing features. It is a very complex enzyme and is associated with eight other proteins to form the pol III holoenzyme. (The term holoenzyme refers to an enzyme that contains several different subunits and retains some activity even when one or more subunits is missing.) Pol III is similar to pol I in that it has a requirement for a template and a primer but its substrate specificity is much more limited. For a template pol III cannot act at a nick nor can it unwind a helix and carry out strand displacement. The latter deficiency means that an auxiliary system is needed to unwind the helix ahead of a replication fork. Pol III, like pol I, possesses a 3 5 exonuclease activity, which performs the major editing function in DNA replication. Polymerase III also has a y exonuclease activity, but this activity does not seem to play a role in replication. 

Errors and Repair. Errors occurring during replication could lead to deleterious mutations. However, many errors are corrected by enzyme activities associated with the complex at the replication fork. The error rate is thus kept at a very... 

Bacterial family C polymerases are the major chromosomal replicative enzyme (Kornberg and Baker, 1992). Like other replicative polymerases, the holoenzyme interacts with other proteins and forms a large multisubunit complex consisting of at least 10 subunits (Kornberg and Baker, 1992). The a-subunit contains the DNA polymerase activity that is tightly associated with the e-subunit, which contains a 3 -5 exonuclease activity (Kelman and O Donnell, 1995). 

The emergence of HIV drng resistance against Pis is associated with accumulation of numerous mutations (mainly) in the HIV PR. The mutation rate of the HIV is very high due to the lack of proofreading activity of the viral reverse transcriptase, and the rapid viral replication in infected persons lead to a selection of viral species resistant to the virostatics that are currently available. The pattern of mutations associated with viral resistance is very complex, and a proper description of the mechanism of resistance development is outside the scope of this chapter (see Weber, 2009 and the references therein). The mutations are selected not only in the protease substrate-binding cleft, but also outside the active site of the enzyme (Figure 3.3). 

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