Isolation of cDNA clones encoding the β isozyme of human DNA topoisomerase II and localisation of the gene to chromosome 3p24

Abstract

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, designatec topoisomerase IIα (170 kDa form) and topoisomerase IIβ (180 kDa form). We have isolated cDNA clones encoding the β isozyme from a human B-cell library. The proposed coding region for the topoisomerase IIβ protein is 4,863 nucleotides long and would encode i polypeptide with a calculated M, of 182,705. The predicted topoisomerase IIβ protein sequence show; striking similarity (72% identical residues) to that of the human α isozyme, and homology to topoisomerase II proteins from Drosophila , yeast and bacteria. Region of greatest amino acid sequence divergence lie at the extreme N-terminus and over a C-terminal domain comprising −25% of the total protein. We have quantified the level of topoisomerase IIβ 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 IIα mRNA. Topoisomerase IIβ mRNA was expressed inhaemopoietic, epithelial and fibroblast cell lines, although of 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 IIα and β genes. We have localised the gene encoding topoisomerase IIβ protein to chromosome 3p24 in the human genome.