Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 113

Bacteria also have more than one type II topoisomerase. Because type II topoisomerases can break both strands and pass two other DNA strands through the break, they can either separate two linked circular DNA molecules or link them up. Linkage sometimes happens after replication or recombination. One major type II topoisomerase in E. coli, Topo IV (see above), decatenates daughter chromosomes after DNA replication, releasing the major source of cohesion between the chromosomes and allowing them to be segregated into the daughter cells. While most type II topoisomerases remove negative supercoils, a special type II topoisomerases in bacteria, called gyrase, plays an essential role in adding negative supercoils. Gyrase acts by first wrapping the DNA around itself and then cutting the two strands before passing another part of the DNA through the cuts, thereby introducing two negative supercoils. Adding negative supercoils increases the stress in the DNA and thus requires energy; hence, gyrase needs ATP for this reaction.

The gyrase of E. coli is made up of four polypeptides, two of which are encoded by the gyrA gene and two of which are encoded by gyrB. These genes were first identified by mutations that make the cell resistant to antibiotics that affect gyrase (Table 1.2 and Box 1.2). The GyrA subunits seem to be responsible for breaking the DNA and holding it as the strands pass through the cuts. The GyrB subunits have the ATP site that furnishes the energy for the supercoiling.

Table 1.2 Antibiotics that block replication

Antibiotic	Source	Target
Trimethoprim	Chemically synthesized	Dihydrofolate reductase
Hydroxyurea	Chemically synthesized	Ribonucleotide reductase
5-Fluorodeoxyuridine	Chemically synthesized	Thymidylate synthetase
Nalidixic acid	Chemically synthesized	gyrA subunit of gyrase
Novobiocin	Streptomyces sphaeroides	gyrB subunit of gyrase
Mitomycin C	Streptomyces caespitosus	Cross-links DNA