Identification of the functional elements in the promoter region of human DNA topoisomerase IIIβ gene

In this study, we have isolated and characterized the promoter region of the human DNA topoisomerase IIIβ (hTOP3β) gene. The 5′ RACE assay showed a short exon 1 encoding only the 35-bp untranslated region and suggested the presence of multiple transcription initiation sites. The hTOP3β gene promoter lacks a canonical TATA box or initiation element and is moderately high in GC content. Transient expression of a luciferase reporter gene under the control of serially deleted 5′-flanking sequence identified an activator element between -141 and -119 upstream of the transcription initiation site and a second regulatory element between -91 and -71. On the basis of scanning mutations of triple nucleotides, we demonstrated that a 5′GGAACC3′ element between -117 and -112 plays a critical role in the up-regulation of the basal transcription activity. Changing the 5′GGAACC3′ sequence leads to markedly reduced promoter activity. Gel mobility shift assays revealed that the 5′GGAACC3′ element is required for DNA binding by the transcription factor complex. These observations lead to the conclusion that the positive regulatory region including the 5′GGAACC3′ core element is essential for efficient expression of the hTOP3β gene as well as for the binding of as yet unidentified regulatory factor(s).