E. coli polymerase

A number of different DNA polymerase molecules engage in DNA replication. These share three important properties (1) chain elongation, (2) processivity, and (3) proofreading. Chain elongation accounts for the rate (in nucleotides per second) at which polymerization occurs. Processivity is an expression of the number of nucleotides added to the nascent chain before the polymerase disengages from the template. The proofreading function identifies copying errors and corrects them. In E coli, polymerase III (pol III) functions at the... 

Polymerases of Class B. Although E. coli polymerase II is a member of this family, relatively little is known about its function. It may participate in DNA repair in the "SOS" response (Section E).265... 

These three levels were demonstrated very nicely in the analysis of the primary structure of the large subunits of the RNA polymerases of H. halobium and H. morrhuae (Leffers et al., 1989). Analysis of the gene cluster coding for these subunits identified the amino acid sequences of four polypeptides B", B, A, and C. When these sequences were aligned with the sequences of known RNA polymerases, the B" + B subunits fitted the eukaryotic polymerase IIB subunit of yeast and the (3 subunit of the E. coli polymerase, where they... 

DNA polymerases (like RNA polymerases) are unique among enzymes in that the choice of substrate is determined by the template. E. coli polymerase can copy eukaryotic DNA and animal polymerases can copy bacterial DNA sequences given the appropriate DNA template. This means that DNA sequences, irrespective of their origin, can be accurately copied using E. coli DNA Pol I and this is the basis of the primed-synthesis DNA sequencing methods. 

Transcription time. What is the minimum length of time required for the synthesis by E. coli polymerase of an mRNA encoding a 100-kd protein ... 

DNA polymerases can be classified into at least three families based on their amino acid sequences [28]. Based on the sequence homologies to E. coli polymerases I, II, and III, the families A, B, and C are defined, respectively. Family B-type DNA polymerases such as T4 phage DNA polymerase, P. Juriosus DNA polymerase or bacteriophage (p29 DNA polymerase are usually monomeric enzymes and possess a 5 3 polymerase activity and a proofreading 3 5 exonuclease activity. Polymerase activity and exonuclease activity... 

The known in-vitro properties of DNA polymerizing enzymes do not provide an explanation of the in-vivo initiation of DNA synthesis (Kornberg, 1969 DeLucia and Cairns, 1969). Consider in-vitro polymerization of deoxyribonucleoside triphosphates by E, coli DNA polymerase. This enzyme catalyzes the addition of deoxyribonucleoside triphosphates to the 3 -hydroxyl terminus of a primer DNA. Such synthesis occurs only in the direction of 5 to 3 , and in all cases studied there is an absolute requirement for DNA template (Kornberg, 1969). The action of E, coli polymerase is illustrated in Figure 4. A DNA template must have an available 3 -hydroxyl terminated strand which can serve as a primer for the initiation of synthesis, and it is assumed that polymerase is bound to an area of the template strand near the 3 -end of the primer. The deoxynucleoside triphosphate is bound adjacent to the 3 -hydroxyl group of the terminal nucleotide to form a base pair with the template. When the correct base pair is formed the polymerase catalyzes a nucleophilic attack by the 3 -hydroxyl group of the primer on the a-phosphorus of the triphosphate. A phosphodiester bond is formed with the subsequent release of pyrophosphate (Fig. 4). 

The importance of the a-factor in determining transcription specificity is illustrated by the events in T4 phage. One minute after infection of E. coli by T4 phage the a units of the E. coli polymerase are inactivated. A few minutes later, the o factor disappears altogether and the subunit is electrophoretically altered. New initiation factors are made. Thus, virus infection is elicited simply by subverting the normal process of gene expression—in some cases by modify-... 

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