Topo I Background Information Topoisomerase I (Topo I) is an important enzyme for Arabidopsis thaliana that mediates DNA replication and chromosome condensation. Disruption of Arabidopsis Topo I gene affects phyllotaxis and plant architecture. The divergence angles and internode lengths between two successive flowers are more random in a Topo I mutant than in wild type plants. DNA topoisomerases are a class of enzymes involved in the regulation of DNA supercoiling. Type I topoisomerases change the degree of supercoiling of DNA by causing single-strand breaks and religation, whereas type II topoisomerases (such as bacterial gyrase) cause double-strand breaks. The different roles of DNA topoisomerase I and II may indicate an opposing pair of roles in the regulation of DNA supercoiling. Both activities are crucial during DNA transcription and replication, when the DNA helix must be unwound to allow proper function of large enzymatic machinery.
