Replication polymerases role

The enzyme, which uses unwound, single-stranded DNA as a template, is called a DNA polymerase. There are three distinct DNA polymerases in E. coli DNA polymerase I, II, and III. DNA polymerase I is the most abundant, and DNA polymerase III the least abundant. These two enzymes have important roles in the overall process of DNA replication. The role of DNA polymerase II has not yet been clearly established. 

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. 

Strauss, B. S., Roberts, R., Francis, L., and Pouryazdanparast, P. (2000). Role of the dinB gene product in spontaneous mutation in Escherichia coli with an impaired replicative polymerase./. Bacteriol. 182, 6742-6750. 

Lichy, J. H., Field, J., Horwitz, M. S., and Hurwitz, J., 1982, Separation of the adenoviral terminal protein precursor from its associated DNA polymerase Role of both proteins in the initiation of adenovirus DNA replication, Proc. Natl. Acad. Sci. USA 79 5225. 

Enzymes in viruses We have stated that virus particles do not carry out metabolic processes. Outside of a host cell, a virus particle is metabolically inert. However, some viruses do contain enzymes which play roles in the infectious process. For instance, many viruses contain their own nucleic acid polymerases which transcribe the viral nucleic acid into messenger RNA once the infection process has begun. The retroviruses are RNA viruses which replicate inside the cell as DNA intermediates. These viruses possess an enzyme, an RNA-dependent DNA popo called reverse transcriptase, which transcribes the information in the incoming RNA into a DNA intermediate. It should be noted that reverse transcriptase is unique to the retroviruses and is not found in any other viruses or in cells. 

Genetic recombination arises by exchange of homologous segments of DNA between viral genomes, most often during the replication process. The enzymes involved in recombination are DNA polymerases, endonucleases, and ligases, which also play a role in DNA repair and synthesis processes. 

The counterpart of DNA polymerases in replication is RNA polymerases in transcription. Just as there are several DNA polymerases in vertebrate cells, so there are several RNA polymerases. To be precise, there are three of them. The different RNA polymerases are associated with three of the classes of RNA molecules found in vertebrate cells. Specifically, RNA polymerase I is responsible for the synthesis of the precursors of most rRNAs. RNA polymerase II plays the same role for the precursors of mRNA. Finally, RNA polymerase III is responsible for the synthesis of the precursors to the tRNAs as well as a few other small RNA molecules. Note here that I have specifically referred to precursors of these classes of RNA molecules. The initial products of the action of the RNA polymerases undergo further metabolism to yield the mature, functional products. 

A large portion of the above mentioned genes correspond to RNA and DNA polymerases. A number of data (Suttle and Ravel, 1974 Lazcano et al, 1988 1992 Frick and Richardson, 2001) suggest that a simplified replicating enzymatic repertoire - as well as a simplified version of protein synthesis - might be possible. From all this, the idea that a single polymerase could play multiple roles as a DNA polymerase, a transcriptase, and a primase, is conceivable in the very early cells (Luisi et al., 2002). 

Figure 4.7. a schematic presentation of the role of protease in processing of HIV gag (structural protein) and polymerase pol essential for producing infectious virus. With inactive HIV protease, the virus generated is immature and hence not infectious. The i indicates where HIV protease cleavage occurs in producing the pol gene product essential for viral replication. 

Yet another polymerase, DNA polymerase e, replaces DNA polymerase S in some situations, such as in DNA repair. DNA polymerase e may also function at the replication fork, perhaps playing a role analogous to that of the bacterial DNA polymerase I, removing the primers of Okazaki fragments on the lagging strand. 

Cairns and De Lucia isolated a mutant strain of E. coli that had only about 1% of the DNA Poll activity found in wild-type cells, yet the strain replicated its DNA at a normal rate. Explain how this discovery was important in understanding the role of the different DNA polymerases in replication and repair. 

This chapter will review the current understanding of the nucleotide incorporation cycle by DNA polymerases and the mechanisms employed by DNA polymerases to replicate DNA accurately. It includes a review of more recent and stimulating work that explores the mechanochemistry of DNA polymerases and their role as force generators and molecular motors. Although additional activities are present on many polymerases (e.g., 5 -3 exonuclease [family A], a 3 -5 exonuclease [family A and B],... 

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