Topoisomerases are a class of enzymes that manage the complex topological challenges of DNA, such as tangles and supercoils, which naturally arise during fundamental cellular processes. Topoisomerase IV is a specific type of topoisomerase, primarily identified in bacteria, where it performs a significant role in maintaining the integrity and proper functioning of the bacterial genome.
What Topoisomerase IV Does
Topoisomerase IV performs two primary functions within bacterial cells. Its most well-known role is decatenation, the process of unlinking interlinked DNA circles, particularly after DNA replication. During replication, the two newly formed circular DNA strands become intertwined, and these links must be separated for the daughter cells to receive a complete chromosome.
This enzyme achieves decatenation by creating a transient double-strand break in one DNA molecule. It then passes the other interlinked DNA segment through this temporary break. Following the passage of the DNA segment, Topoisomerase IV rejoins the broken DNA strands, effectively unlinking the two DNA molecules. This precise action allows for the proper segregation of chromosomes into daughter cells during cell division.
Another function of Topoisomerase IV is the relaxation of positive supercoils in DNA. As DNA unwinds during replication, positive supercoils can build up ahead of the replication fork, creating tension that would impede the process. Topoisomerase IV, along with DNA gyrase, works to relieve this topological strain. While it relaxes positive supercoils, Topoisomerase IV does not introduce negative supercoils, a distinct function of DNA gyrase.
Why Topoisomerase IV is Critical
The proper functioning of Topoisomerase IV is necessary for bacterial survival. Without its activity, bacteria are unable to complete the separation of their newly replicated chromosomes. This inability to decatenate the intertwined DNA leads to severe issues in cell division, ultimately preventing the bacteria from multiplying.
Its role in unlinking daughter chromosomes after replication makes it an indispensable enzyme for bacterial growth. If Topoisomerase IV activity is impaired or absent, the bacterial cell cannot divide, leading to cell death. Its indispensable role in bacterial chromosome segregation makes it an appealing target for antimicrobial strategies.
Targeting Topoisomerase IV with Antibiotics
Certain classes of antibiotics specifically target bacterial Topoisomerase IV to exert their antimicrobial effects. Quinolone antibiotics, such as ciprofloxacin and levofloxacin, interfere with this enzyme’s function. These drugs work by trapping Topoisomerase IV in a state where it is bound to DNA, but unable to rejoin the broken DNA strands.
When quinolones interact with Topoisomerase IV, they stabilize the enzyme-DNA complex after the DNA has been cut. This stabilization prevents the enzyme from rejoining the DNA, leading to persistent double-strand breaks in the bacterial chromosome. The accumulation of these unrepaired DNA breaks results in lethal DNA damage and the inhibition of bacterial replication.
Topoisomerase IV is one of two primary targets for quinolone antibiotics in bacteria; the other being DNA gyrase. Targeting both of these enzymes simultaneously enhances the effectiveness of quinolone drugs, as they play complementary roles in bacterial DNA management. This dual targeting increases the likelihood of disrupting bacterial growth and leading to bacterial cell death.
How Bacteria Develop Resistance
Bacteria can develop resistance to antibiotics that target Topoisomerase IV through several mechanisms. A common mechanism involves mutations in the genes that encode Topoisomerase IV itself. These mutations frequently occur in specific regions of the genes, known as quinolone resistance-determining regions (QRDRs).
Mutations within these QRDRs can alter the three-dimensional structure of the Topoisomerase IV enzyme. This structural change can prevent the antibiotic from binding effectively, reducing the drug’s ability to trap the enzyme-DNA complex. As a result, Topoisomerase IV continues its normal function, allowing bacteria to survive and multiply even in the presence of the antibiotic. While other mechanisms like efflux pumps or enzymatic inactivation can contribute, mutations in the enzyme’s binding site are a primary way bacteria overcome Topoisomerase IV-targeting antibiotics.