E. coli DNA Polymerases

The earliest studies of DNA polymerases began in the 1950s with Arthur Kornberg's discovery of DNA polymerase from E. coli. Three DNA polymerases are known in E. coli:

> DNA polymerase I

> DNA polymerase II

> DNA polymerase III

DNA polymerases use deoxyribonulceoside triphosphates (dNTPs) to synthesize DNA sequences. dNTPs are activated compounds which are cleaved to generate a pyrophosphate and a deoxyribonucleoside monophosphate (dNMP) covalent linked into a DNA chain. dNMPs are added into the growing chain exclusively in the 5' to 3' direction. DNA polymerases require a "primer", which is a preexisting nucleic acid annealed at the site where replication is to begin (Figure 24.15). Primers of DNA, as well as RNA, can be extended by a DNA polymerase. In addition to the catalytic activity for polymerizing DNA, many DNA polymerases also contain one or more exonuclease activities. The function of these is described with each enzyme above. DNA polymerases can copy around a circular single-strand template, such as the DNA extracted from small bacteriophages (e.g., X174 or M13), as long as a primer is present. DNA polymerases cannot, however, join the ends. When the template is linear, polymerase copies only to the 5´ end of the template and then it dissociates.

See also: Eukaryotic DNA Polymerases, Other Replication Proteins, Figure 24.8, Table 24.2

INTERNET LINK: DNA Replication Enzymes