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

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Abstract
Nuclear extracts of [chicken] 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 to topoisomerases to DNA. 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 [kilodalton] 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-MW forms is relatively constant in erythrocytes and embryonic fibroblasts. These results imply that the multiple forms are not unique to one tissue setting.