Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 88

DNAs also become joined to each other through the formation of catenanes, where the daughter DNAs become interlinked like the links on a chain. These interlinks can form as a side effect of separating DNA strands during the process of DNA replication (Figure 1.18). As we discuss in “Supercoiling” below, DNA replication introduces a great deal of torsional stress ahead of the DNA replication fork (Figure 1.18A and B). This stress can be transferred across the DNA replication fork into the newly formed DNA strands in twists between the two new daughter strands called precatenanes (Figure 1.18C), because the twists will eventually form catenanes when replication is complete (Figure 1.18D). Once such interlinks are formed, the only way to unlink them is to break both strands of one of the two DNAs and pass the two strands of the other DNA through the break. The break must then be resealed. This double-strand passage, called decatenation, is one of the reactions performed by type II topoisomerases (see below). A type II topoisomerase called topoisomerase IV (Topo IV) plays a major role in removing most of the interlinks between the daughter DNAs in E. coli. The act of removing these links appears to remove one of the major cohesive forces between the daughter chromosomes prior to segregation. One of the major points of regulation appears to be spatial, where the two subunits that make Topo IV are most likely to interact through association with other proteins. In one case, this occurs following replication, when the chromosomes are being translocated across the division septum by FtsK (Figure 1.19). FtsK helps to regulate the decatenation process as it pulls the chromosome to the septum, because one of the subunits of Topo IV interacts with FtsK. Interaction between Topo IV and the FtsK protein puts the enzyme in a very appropriate position for the removal of catenanes just before chromosome segregation. Topo IV is also regulated through an interaction with a protein involved in the condensation of chromosomes following DNA replication (see “Condensation” below).