DNA topoisomerases are known to be involved in regulation of gene expression as it modifies DNA torsional stress in chromatin. To test if the over-expression of DNA topoisomerase I affects gene expression, the enzyme is over-expressed in yeast, Saccharomyces cerevisiae. Recombinant plasmids containing wild type TOPI and mutant topI gene were constructed by using YEp multicopy plasmid vector containing galactose-inducible promoter and transformed into yeast cells. The level of enzyme activity was monitored by addition of tester supercoiled plasmid exogenously in the yeast extracts to compare TOPI and topI expression. Based on the relaxation activity of supercoiled plasmid in cell extracts of various condition, the cells carrying TOPI gene with galactose-inducible promoter was at a higher level when compared to that of topI This result indicates that transiently formed supercoils in vivo may be relaxed and disappeared by the over-produced enzyme. The expression of the HSP82 mRNA in the above cell was reduced by five fold, suggesting its global effect on gene expression. Two dimensional SDS-PAGE analysis has been performed and protein expression patterns were compared between the above strains. As a result, over-expression of DNA topoisomerase I affects expression of many proteins. These data are consistent with the idea that torsional stress is required for transcription.