Other Replication Proteins

In eubacteria and eukaryotes, several types of DNA polymerases have been characterized three in eubacteria (DNA polymerases I, II and III), and five in eukaryotes (DNA polymerases a, 3, 6, e and )). Some of these enzymes, named DNA replicases , are specifically involved in DNA-chain elongation at the replication fork. They have a multi-subunit structure and can prime and perform DNA replication in a processive way when they are associated with the other replicative proteins. In eubacteria, only one DNA replicase has been isolated (DNA polymerase III), whereas several DNA replicases co-exist in eukaryotes DNA polymerases a, 6 and e, which are essential for the replication of nuclear DNA, and DNA polymerase y, which is responsible for the replication of the mitochondrial genome. The other eubacterial and eukaryotic DNA polymerases are monomeric and are preferentially involved in mechanisms which require replication of short DNA fragments, in the course of either DNA repair (DNA polymerases I and II from E. coli, eukaryotic DNA polymerase 3), or DNA replication (maturation of Okasaki fragments by E. coli DNA polymerase I). 

DNA Replication (from Chapter 4), E. coli DNA Polymerases, Eukaryotic DNA Polymerases, Other Replication Proteins, Topoisomerases... 

See also Other Replication Proteins, Replication Fork... 

See also E. coli DNA Polymerases, Other Replication Proteins... 

DNA Replication Overview Replication Fork E. coli DNA Polymerases Eukaryotic DNA Polymerases Other Replication Proteins Topoisomerases Uracil-DNA N-Glycosylase Replication Complexes Initiation of DNA Replication Replication of Linear Genomes Fidelity of DNA Replication... 

Other HIV proteins include regulator of viral expression (Rev), negative effectors (Nef), viral protein R (Vpr), viral protein U (Vpu), viral infectivity factor (Vif) and transactivator protein (Tat). These proteins are instrumental in viral mRNA expression, viral replication and transactivation, viral release and maturation, viral infection, and maintenance of viral transcript activation and expression, respectively (Tripathi and Agrawal 2007). 

Two other protein complexes also function in eukaryotic DNA replication. RPA (replication protein A) is a eukaryotic single-stranded DNA-binding protein, equivalent in function to the E. coli SSB protein. RFC (replication/actor (J) is a clamp loader for PCNA and facilitates the assembly of active replication complexes. The subunits of the RFC complex have significant sequence similarity to the subunits of the bacterial clamploading (y) complex. 

We look at E. coli DNA polymerase I as an example of how biochemical and genetic studies are used to characterize a DNA replication enzyme. Then we consider the other major proteins involved in DNA replication in E. coli before examining the proteins that participate in the synthesis of DNA in different types of organisms. 

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). 

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