Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 111

The supercoiling of DNA in the cell is modulated by topoisomerases (see Wang, Suggested Reading). Topoisomerases are discussed above, but not the molecular details of these enzymes. All organisms have these proteins, which manage to remove the supercoils from a circular DNA without permanently breaking either of the two strands. They perform this feat by binding to DNA, breaking one or both of the strands, and passing the DNA strands through the break before resealing it. As long as the enzyme holds the cut ends of the DNA so that they do not rotate, this process, known as strand passage, either introduces or removes supercoils in DNA.

The topoisomerases are classified into two groups, type I and type II (Figure 1.27). These two types differ in how many strands are cut and how many strands pass through the cut. The type I topoisomerases cut one strand and pass the other strand through the break before resealing the cut. The type II topoisomerases cut both strands and pass two other strands from somewhere else in the DNA, or even another DNA, through the break before resealing it. This basic difference changes how supercoiling is affected by these enzymes, as shown in Figure 1.27.