Cloning and characterization of the 5'-flanking region for the human topoisomerase III gene.

The human DNA topoisomerase III (hTOP3) gene encodes a topoisomerase homologous to the Escherichia coli DNA topoisomerase I subfamily. To understand the mechanisms responsible for regulating hTOP3 expression, we have cloned the 5'-flanking region of the gene coding for the hTOP3 and analyzed its promoter activity. The presence of a single transcription initiation site was suggested by primer extension analysis. The hTOP3 gene promoter is moderately high in GC content and lacks a canonical TATA box, suggesting that hTOP3 promoter has overall similarity to promoters of a number of housekeeping genes. Examination of the promoter sequence indicated the presence of four Sp-1 consensus binding sequences and a putative initiator element surrounding the transcription initiation site. Transient expression of a luciferase reporter gene under the control of serially deleted 5'-flanking sequences revealed that the 52-base pair region from -326 to -275 upstream of the transcription initiation site includes a positive cis-acting element(s) for the efficient expression of hTOP3 gene. On the basis of gel mobility shift and supershift assays, we demonstrated that both YY1 and USF1 transcription factors can bind to the 52-base pair region. When HeLa cells were transiently transfected with a mutant construct which had disabled both YY1- and USF1-binding sites, the luciferase activity was greatly reduced, suggesting that these binding elements play a functional role in the basal activation of the hTOP3 promoter. Transfection studies with mutations that selectively impaired YY1 or USF1 binding suggested that both YY1 and USF1 function as activators in the hTOP3 promoter.