Human DNA topoisomerase I is the best studied of the DNA topoisomerase family. It catalyzes the relaxation of both positive and negative supercoiled DNAs without the requirement of energy. In addition to DNA replication and transcriptional activation, DNA topoisomerase I also plays a major role in pre-mRNA splicing, cell cycle and other gene regulatory pathways during cell growth and development. Tyrosine 723 was identified as an active site for the DNA binding activity of DNA topoisomerase I. The covalent intermediate of topo I and DNA complex includes nucleophilic attack by the O4-oxygen of tyrosine 723 on a phosphester linkage in the DNA. Mutation from tyrosine to phenylalanine at position 723 preferentially binds the supercoiled DNA rather than relaxed DNA in the mixture of supercoiled and relaxed DNAs. But mutation at tyr⁷²³ neither affects its kinase activity that phosphorylates splicing factors of SR protein family nor its transcription activity of class II genes in vitro.

Clear and colorless frozen liquid solution

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