Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 61

Enzymes that degrade DNA strands by breaking the phosphodiester bonds are just as important in replication as the enzymes that polymerize DNA by forming phosphodiester bonds between the nucleotides. These bondbreaking enzymes, called nucleases, can be grouped into two major categories. One type can initiate breaks in the middle of a DNA strand and so are called endonucleases, from a Greek word meaning “within,” and the other type can remove nucleotides only from the ends of DNA strands and so are called exonucleases, from a Greek word meaning “outside.” A special type of endonuclease activity, called a flap endonuclease activity, is involved in primer removal by DNA polymerase I. The flap endonuclease activity appears to be common to all organisms for removing RNA primers. In E. coli, DNA polymerase I displaces the RNA primer, making a flap-like structure, and then the flap endonuclease activity of Pol I cleaves away the oligonucleotide as indicated (Figure 1.9). The exonucleases can be subdivided into two groups. Some exonucleases can degrade only from the 3′ end of a DNA strand, degrading DNA in the 3′-to-5′ direction. These are called 3′ exonucleases; one example of their activity is their role in the editing function associated with DNA polymerases I and III, which is discussed below. Other exonucleases, called 5′ exonucleases, degrade DNA strands only from the 5′ end.