Topoisomerase I is the predominant nuclear protein from avian erythrocytes that can be covalently linked to DNA.

Nuclear extracts of erythrocytes contain proteins which stably or possibly covalently bind to DNA. These proteins can be detected by an assay which was originally developed to quantify stable binding of topoisomerases to DNA [Trask, DiDonato & Muller (1984) EMBO J. 3, 671-676]. In this report, we show that the number of activities detected by this assay in crude extracts of nuclei is limited predominantly to various forms of topoisomerase I. One form, a 50 kDa protein, copurifies with histone H1. Western blotting experiments suggest that the 50 kDa topoisomerase exists in chromatin along with the 105 kDa form. In addition, the ratio between the high and low-Mr forms is relatively constant in erythrocytes and embryonic fibroblasts. These results imply that the multiple forms are not unique to one tissue setting.