To better understand the role of topoisomerase activity in relieving transcription-induced supercoiling, yeast genes encoding ribosomal RNA were visualized in cells deficient for either or both of the two major topoisomerases. In the absence of both Topoisomerase I and Topoisomerase II activity, processivity was severely impaired and polymerases were unable to transcribe through the 6.7 kb gene. Loss of Top1 resulted in increased negative superhelical density (2-6 times the normal value) in a significant subset of rDNA genes, as manifested by regions of DNA template melting. The observed DNA bubbles were not R-loops and did not block polymerase movement, since genes with DNA template melting showed no evidence of slowed elongation. Inactivation of Top2, however, resulted in characteristic signs of slowed elongation in rDNA genes, suggesting that Top2 alleviates transcription-induced positive supercoiling. Together the data indicate that torsion in front of and behind transcribing Pol I has different consequences and different resolution. Positive torsion in front of the polymerase induces supercoiling (writhe) and is largely resolved by Top2. Negative torsion behind the polymerase induces DNA strand separation and is largely resolved by Top1.
|Evidence ID||Analyze ID||Interactor||Interactor Systematic Name||Interactor||Interactor Systematic Name||Type||Assay||Annotation||Action||Modification||Phenotype||Source||Reference||Note|
|Evidence ID||Analyze ID||Gene||Gene Systematic Name||Gene Ontology Term||Gene Ontology Term ID||Qualifier||Aspect||Method||Evidence||Source||Assigned On||Reference||Annotation Extension|
|Evidence ID||Analyze ID||Gene||Gene Systematic Name||Phenotype||Experiment Type||Experiment Type Category||Mutant Information||Strain Background||Chemical||Details||Reference|
|Evidence ID||Analyze ID||Regulator||Regulator Systematic Name||Target||Target Systematic Name||Experiment||Conditions||Strain||Source||Reference|