HUMAN-CELLS EXPRESS 2 DIFFERENTIALLY SPLICED FORMS OF TOPOISOMERASE-II-BETA MESSENGER-RNA

Screening of a human B-cell cDNA library with a topoisomerase IIbeta g ene-specific probe revealed the presence of two distinct forms of topo isomerase IIbeta cDNA. One form (designated topoisomerase IIbeta-1), r epresenting the majority of the clones, would encode the topoisomerase IIbeta amino acid sequence reported recently [Jenkins, J.R. et al. (1 992) Nucleic Acids Res., 20, 5587-5592]. The second form (designated t opoisomerase IIbeta-2) would encode a protein containing an additional 5 amino acids inserted after Valine-23 of the topoisomerase IIbeta-1 protein sequence. The topoisomerase IIbeta-1 and beta-2 mRNAs were bot h widely expressed in human cell lines and tissues. Topoisomerase IIbe ta-2 mRNA was expressed at a lower level than that of the beta-1 form, but the relative expression of the two forms varied in different cell types. Analysis of genomic DNA clones revealed that the two forms of topoisomerase IIbeta mRNA arose via differential splicing. These data indicate that in addition to the closely related topoisomerase IIalpha and beta isozymes, there are two forms of topoisomerase IIbeta mRNA w idely expressed in human cells.