ISOLATION OF CDNA CLONES ENCODING THE BETA-ISOZYME OF HUMAN DNA TOPOISOMERASE-II AND LOCALIZATION OF THE GENE TO CHROMOSOME 3P24

Topoisomerases catalyse the interconversion of topological isomers of DNA and have key roles in nucleic acid metabolism. Human cells express two distinct type II topoisomerase isozymes, designated topoisomerase IIalpha (170 kDa form) and topoisomerase IIbeta (180 kDa form). We have isolated cDNA clones encoding the beta isozyme from a human B-cell library. The proposed coding region for the topoisomerase IIbeta protein is 4,863 nucleotides long and would encode a polypeptide with a calculated M(r) of 182,705. The predicted topoisomerase IIbeta protein sequence shows striking similarity (72% identical residues) to that of the human alpha isozyme, and homology to topoisomerase II proteins from Drosophila, yeast and bacteria. Regions of greatest amino acid sequence divergence lie at the extreme N-terminus and over a C-terminal domain comprising approximately 25% of the total protein. We have quantified the level of topoisomerase IIbeta mRNA in a panel of human tumour cell lines of different origin using an RNase protection assay, and compared the level to that of topoisomerase IIalpha mRNA. Topoisomerase IIbeta mRNA was expressed in haemopoietic, epithelial and fibroblast cell lines, although to different extents, with U937 cells (promonocytic leukaemia) showing a particularly high level. There was no obvious relationship in terms of level of expression between the topoisomerase IIalpha and beta genes. We have localised the gene encoding topoisomerase IIbeta protein to chromosome 3p24 in the human genome.