DNA polymerase III

B Guenther, R Onrust, A Sail, M O Donnell, J Kuriyan. Crystal structure of the 5 subunit of the clamp-loader complex of E. coli DNA polymerase III. Cell 91 335-345, 1997. 

DNA polymerase III has all the enzymatic activities of DNA polymerase I. A subunit of the enzyme is the product of the dna E gene. Temperature-sensitive mutations of this gene testify to the importance... 

Figure 9.3 A single unit of DNA polymerase III complex synthesizes both new strands of DNA, one continuously and the other in short pieces. Deoxynucleotide additon to die daughter strands is indicated by vertical lines across the strands.

DNA polymerases from several different animal cells have been isolated and studied. The three DNA polymerases of animal cells, called a, p, and y, can be distinguished by their molecular weights, template specificity, and sensitivity to sulfhydryl reagents. Table 14.3 compares the three in regard to these differences. DNA polymerase a is probably the most important for DNA replication. This enzyme shares many functional properties with DNA polymerase III of E. coli ... 

Bridges, B.A., Woodgate, R., Ruiz-Rubio, M., Sharif, F., Sedgwick, S.G. and Huhschere, U. (1987). Current understanding ofUV-induced base pair substitution mutation inE. coli with particular reference to the DNA polymerase III complex. Mutation Res. 181 219-226. 

DNA polymerase III begiiis synthesizing DNA in the 5 ->3 direction, beginning at the 3 end of each RNA primer. The newly synthesized strand is complementary and antiparaUd to the parental strand used as a template. This strand can be made continuously in one long piece and is known as the leading strand. ... 

The lagging strand is synthesized discontinuously as a series of small fragments (about 1,000 nucleotides long) known as Okazaki fragments. Each Okazaki fragment is initiated by the synthesis of an RNA primer by primase, and then Completed by the synthesis of DNA using DNA polymerase III. Each fr ment is made in the 5 - 3 direction. 

Synthesis of DNA Leading strand Lading strand (Okazaki fragments) DNA polymerase III DNA polymerase lU DNA polymerase 6 DNA polymerase a... 

DNA polymerase III is the main DNA replicating enzyme and consequently is involved in duplication of both the leading and the lagging strands. Besides the deoxyribonu-cleoside triphosphates as substrates, it requires a single-stranded DNA template and an RNA primer. The RNA primer is synthesised according to the DNA sequence at each replication fork. It is DNA polymerase I that is... 

Figure 11-2. The prokaryotic DNA replication fork. A schematic representation of semi-conservative replication of DNA by different mechanisms on the leading and lagging strands by DNA polymerase III (DNA pol III) is shown. Other enzymes and accessory proteins that participate in initiation, elongation, and ligation phases of the process are indicated, with DNA pol I depicted as having just dissociated from a completed Okasaki fragment. SSBs, single-stranded DNA binding proteins.

DNA polymerase III (DNA pol HI), the main DNA polymerase of Escherichia coli, synthesizes DNA continuously on the leading strand and discon-tinuously on the lagging strand. 

Ter (Replication termination) xthA AP endonuclease holE DNA polymerase III subunit... 

A search for other DNA polymerases led to the discovery of E. coli DNA polymerase II and DNA polymerase III in the early 1970s. DNA polymerase II... 

Polymerization subunit only DNA polymerase II shares several subunits with DNA polymerase III, including the (3, y, 8, 8, x, and ip subunits (see Table 25-2),... 

DNA polymerase III is much more complex than DNA polymerase I, having ten types of subunits (Table 25-2). Its polymerization and proofreading activities reside in its a and e (epsilon) subunits, respectively. The 6 subunit associates with a and e to form a core polymerase, which can polymerize DNA but with limited processivity. Two core polymerases can be linked by... 

DNA polymerase III can polymerize DNA, but with a much lower processivity than one would expect for the organized replication of an entire chromosome. The necessary increase in processivity is provided by the addition of the J8 subunits, four of which complete the DNA polymerase III holoenzyme. The J3 subunits associate in pairs to form donut-shaped structures that encircle the DNA and act like clamps (Fig. 25-10b). Each dimer associates with a core subassembly of polymerase III (one dimeric clamp per core subassembly) and slides along the DNA as replication proceeds. The J8 sliding clamp prevents the dissociation of DNA polymerase III from DNA, dramatically increasing processivity—to greater than 500,000 (Table 25-1). 

Most cells have several DNA polymerases. In E. coli, DNA polymerase III is the primary replication enzyme. DNA polymerase I is responsible for special functions during replication, recombination, and repair. 

DNA polymerase III Exonuclease I Exonuclease Vii Reel nuclease Exonuclease X DNA ligase... 

---