Cloning and characterization of the 5'-flanking sequence for the human DNA topoisomerase II beta gene.

Mammalian cells express two isoforms of type II DNA topoisomerase which are the intracellular targets of many structurally diverse antineoplastic agents. The levels of topoisomerase II isozymes are important determinants for the sensitivity of cells to the cytotoxicity of drugs that target topoisomerase II. To investigate whether the expression of topoisomerase II isoforms is coordinated and the mechanisms governing their expression in the context of drug resistance, the 5'-flanking sequence for the gene of human topoisomerase IIbeta isoform was cloned and characterized. The 5'-flanking region has a very high GC content and contains no canonical TATA box element. Two separate transcriptional start sites are located to an adenine and a guanine, 193 and 89 nucleotides, respectively, upstream from the ATG translation initiation codon. Except for a small region immediately upstream of the translation initiation codon, there is no obvious sequence homology between the 5'-flanking sequences of human topoisomerase IIbeta gene and the previously described alpha gene. Transient expression assays with different 5'- and 3'-deletions of the 5'-flanking region of the topoisomerase IIbeta gene have delineated regions important for transcriptional regulation of the gene. Interestingly, sequences within the first intron also contribute to the promoter activity. Gel mobility shift studies demonstrate that protein factors from the nuclear extracts can bind specifically to the downstream elements and may participate in transcriptional regulation.