Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 108

Supercoiling is one of the mechanisms that help compact and organize the chromosome. Supercoiling also affects the expression level of many genes. However, supercoiling will be lost if one of the strands of the DNA is cut, thereby allowing the strands to rotate around each other. The phosphodiester bond connecting the two deoxyribose sugars on the other strand serves as a swivel and rotates, resulting in relaxed (i.e., not supercoiled) DNA. A DNA with a phosphodiester bond broken in one of the two strands is said to be nicked. A variety of experiments suggest that the nucleoid is packaged in subregions that constrain supercoiling. Topological barriers would prevent the entire chromosome from losing supercoiling when there is a nick in the genome. Figure 1.26 illustrates supercoiling. In this example, the ends of a DNA molecule have been rotated in opposite directions, and the DNA has become twisted up on itself to relieve the stress. The DNA remains supercoiled as long as its ends are constrained and so cannot rotate, and a circular DNA has no free ends that can rotate. A linear DNA will not maintain supercoiling unless regions flanking the supercoiling are somehow otherwise constrained. A break or nick in a circular DNA should relax the whole DNA unless portions of the molecule are periodically attached to barriers that prevent rotation of the strands. Through the examination of the expression level of over 300 genes that are sensitive to supercoiling following the introduction of breaks in the chromosome, it has been estimated that the topologically isolated loops are about 10 kb in size. This is in good agreement with domain size as determined by directly measuring the lengths of loops under the microscope (see Postow et al., Suggested Reading). Although the exact mechanism or mechanisms that restrict topology are unresolved, this work indicates that the barriers to rotation are not fixed at certain places in the chromosome.

Figure 1.26 (A) Supercoiled DNA. (B) Twisting of the ends in opposite directions causes linear DNA to wrap up on itself. The supercoiling is lost if the ends of the DNA are not somehow constrained. (C) A break, or nick, in one of the two strands of a circular DNA relaxes the supercoils.