Figure 4.

Bacterial type II topoisomerases are essential but potentially toxic enzymes. The balance between enzyme-mediated DNA cleavage and religation is critical for cell survival. If the level of gyrase-mediated DNA cleavage decreases, rates of DNA replication slow and impair cell growth (left). If the level of topoisomerase IV-mediated DNA cleavage decreases, cells are not able to untangle daughter chromosomes and ultimately die of mitotic failure (left). If the level of gyrase- or topoisomerase IV-mediated DNA cleavage becomes too high (right), the actions of DNA tracking systems can convert these transient complexes to permanent double-stranded breaks. The resulting DNA breaks initiate the SOS response and other DNA repair pathways and can lead to cell death.