- Topoisomerase IV is a bacterial enzyme that plays a crucial role in DNA replication and chromosome segregation.
- As a type II topoisomerase, it functions alongside DNA gyrase to regulate DNA topology, particularly by relieving supercoiling and resolving tangles that arise during replication. While DNA gyrase introduces negative supercoiling, topoisomerase IV is primarily responsible for unlinking interlinked DNA molecules, ensuring proper chromosome separation.
- Topoisomerase IV is composed of two subunits, ParC and ParE, which work together to catalyze the relaxation of supercoiled DNA and the decatenation (unlinking) of daughter chromosomes after replication.
- ParC – Contains the DNA cleavage and religation domain, allowing the enzyme to transiently break DNA strands and reseal them.
- ParE – Houses the ATPase domain, which provides the energy necessary for strand passage and enzyme function.
- This enzyme belongs to the type II topoisomerase family, meaning it functions by cutting both DNA strands, passing another strand through the break, and then resealing the strands to maintain genomic integrity.
- Topoisomerase IV primarily performs two essential functions in bacterial cells:
- Relaxing supercoiled DNA – During replication and transcription, DNA can become excessively coiled, hindering polymerase movement. Topoisomerase IV alleviates this stress by introducing transient breaks and allowing DNA relaxation.
- Decatenation of replicated chromosomes – After DNA replication, newly synthesized daughter chromosomes can become interlinked. Topoisomerase IV ensures their proper segregation by cutting and resolving these catenated DNA structures, preventing replication errors and enabling smooth cell division.
- The enzyme functions through an ATP-dependent strand-passage mechanism. It binds to a DNA segment, cleaves both strands, passes another DNA duplex through the gap, and reseals the break. This process prevents chromosome entanglement, ensuring accurate inheritance of genetic material.
- Topoisomerase IV is a major target for fluoroquinolone antibiotics, which inhibit its activity and cause lethal DNA fragmentation in bacteria. While fluoroquinolones primarily target DNA gyrase in Gram-negative bacteria, they preferentially inhibit topoisomerase IV in Gram-positive bacteria.
- By stabilizing the enzyme-DNA complex after cleavage, fluoroquinolones prevent re-ligation, leading to double-strand breaks that result in bacterial cell death. This mechanism makes fluoroquinolones highly effective bactericidal agents, commonly used against respiratory, urinary tract, and soft tissue infections.
- However, bacterial resistance to fluoroquinolones has emerged through mutations in topoisomerase IV genes (parC and parE), as well as the development of efflux pumps that reduce drug accumulation. Understanding the structure and function of topoisomerase IV is critical in antibiotic design and overcoming resistance mechanisms.
