DNA replicase

Replication in E. coli requires not just a single DNA polymerase but 20 or more different enzymes and proteins, each performing a specific task. The entire complex has been termed the DNA replicase system or replisome. The enzymatic complexity of replication reflects the constraints imposed by the structure of DNA... 

Chromosome duplication in eukaryotic cells requires the enzyme telomerase which replicates the chromosome ends, the telomeres, a task that cannot be carried out by DNA replicases. (Telomerase is a ribonucleoprotein where the RNA moiety serves as template for addition of short nucleotides to the 3 end of the chromosome. The telomerase contains reverse transcriptase motifs which are essential for the duplication of the telomeres. For further information see ref. 20.). The cell-cycle clock is tightly coupled to the telomerase clock. ... 

Johnson A, O Donnell M. Cellular DNA replicases components and dynamics at the replication fork. Annu. Rev. Biochem. 2005 74 283-315. 

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). 

A number of enzymes mediate DNA synthesis (BuireU, 1993 Komberg, 1988). These include helicase and topoisomerase (unwinding and provision of the template), primase (synthesis of primer), DNA dependent DNA polymerase (synthesis of polynucleotide chain), and ligase (joining of Okazaki fragments). These enzymes and proteins form DNA replicase system (Johnson and O Donnel, 2005), as depicted in Figure 13.6. 

Yagura T, Kozu T, Seno T (1982) Mouse DNA replicase DNA polymerase associated with a novel RNA polymerase, activity to synthesize initiator RNA of strict size. J Biol Chem 257 11121-11127... 

PSF specifically stimulated the primase activity and, thus, the DNA replicase activity of purified DNA polymerase a-primase. As shown in Fig. 1, the primer synthesis by this enzyme was completely dependent on template, required appropriate concentration of deoxyribonucleoside triphosphate and was markedly stimulated by a very low concentration of PSF (more than 20-fold stimulation was observed at 10 ng/50 pi of PSF). The factor was also effective in stimulating the replicase activity supported by single stranded calf thymus or 0X 174 DNA as template, indicating that primer synthesis in these reactions was also stimulated. However, PSF is not primase itself since the factor neither shows any primer synthesis by itself nor does it stimulate the activity of E. coli DNA polymerase I with poly (dT) as template. A factor with properties similar to PSF has been found in mouse tissue by Yagura, Kozu, and Seno (8) but was not detected in other vertebrates tested by them (9). However, the molecular component of their factor was significantly smaller (63 kDa) than the one described here (8). 

Enzymes have been isolated from the most diverse sources that can support DNA synthesis in a cell-free system. They are called DNA polymerases or DNA replicases. They need, in addition to other cofactors, DNA as primer and deoxyribonucleoside-5 -triphosphates as DNA building blocks. The triphosphates align themselves on the DNA single strand introduced as primer according to the rules of base pairing an... 

Although the exact DNA replicase of animal cells has not been conclusively identified, it is believed that DNA polymerase I (or a) is most likely the DNA replicase. In phytohemagglutinin-stimulated human lymphocytes, polymerase I activity increased with DNA synthesis (Ber-tazzoni et al., 1976). In HeLa cells, DNA polymerase I increased tenfold during the Gi period and decreased after the S period (Chiu and Baril, 1975). In mice, polymerase I increased during liver regeneration (Hecht, 1975). 

Seki, S., and Mueller, G. G., 1975, A requirement for RNA, protein, and DNA synthesis in the establishment of DNA replicase activity in synchronized HeLa cells, Biochim. Biophys. Acta 378 354. 

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