Characterization of the 3' untranslated region of mouse DNA topoisomerase IIalpha mRNA.

Expression of DNA topoisomerase IIalpha protein varies through the cell cycle with its peak in G2/M. This cell-cycle-dependent expression depends on changes in topoisomerase IIalpha mRNA stability as well as promoter activity. We isolated the 3' genomic region of the mouse topoisomerase IIalpha gene and investigated whether or not the 3' untranslated region (UTR) of the topoisomerase IIalpha mRNA participates in the cell-cycle-dependent mRNA stability. Interestingly, genomic- and RT-PCR analyses revealed that the topoisomerase IIalpha 3' UTR is formed via splicing in mouse, but not in human and hamster. Comparison of the mouse 3' region with the human and hamster regions suggests that this mouse-specific splicing has resulted from an accidental acquisition of the consensus 5' splice site. The minority of the non-spliced topoisomerase IIalpha 3' UTR in mouse was confirmed by Northern blot analysis. We performed transient expression assays using luciferase constructs with the mouse topoisomerase IIalpha 3' genomic region, or the major spliced form of the 3' UTR. However, neither construct affected the cell-cycle-dependent expression of the reporter gene driven by the topoisomerase IIalpha promoter. Our results strongly suggest that the mouse topoisomerase IIalpha 3' UTR by itself is not involved in the cell-cycle-dependent mRNA stability.