DNA topoisomerase I

Topoisomerase 1 Dmel_CG6146 is the Drosophila melanogaster ortholog of human DNA topoisomerase I (TOP1), encoded by CG6146. topoisomerase I resolves the topological strain that accumulates ahead of replication forks and transcribing polymerases: it cleaves one strand of the duplex, forming a transient covalent intermediate — the TOP1 cleavage complex (TOP1cc), in which an active-site tyrosine is linked to the 3′ end of the nick — allows the DNA to rotate about the intact strand, and then reseals it. Unlike type II enzymes it needs no ATP, since the energy of the broken phosphodiester bond is stored in the protein–DNA link and recovered on religation. The reaction is essential in both directions of gene expression, and without it transcription generates positive supercoils ahead and negative supercoils behind that stall the polymerase and promote R-loop formation.

The enzyme is one of the most validated antitumor drug targets, for a reason that lies in its intermediate rather than its function. camptothecin-class inhibitors do not block catalysis; they bind at the enzyme–DNA interface and prevent religation, converting a normal transient intermediate into a persistent protein-linked single-strand break. Double-strand breaks arise secondarily, when a replication fork or transcription complex collides with the trapped complex or during its processing — which makes the drugs S-phase selective and makes exposure time, rather than peak concentration, the determinant of cell kill. Cells defend themselves by removing the adduct: tyrosyl-DNA phosphodiesterase 1 (TDP1) hydrolyzes the tyrosyl–DNA bond, and its loss sensitizes cells to these agents, while hereditary TDP1 deficiency causes a neurodegenerative ataxia, showing that trapped complexes accumulate in non-dividing neurons too.

The Drosophila protein is highly conserved with its human counterpart and has served as a tractable genetic and biochemical model for TOP1 function, inhibitor sensitivity and the DNA damage response — a role that depends on that conservation, since the trapping mechanism and its repair are shared across the eukaryotes.