LOCALIZATION OF A HIGH-MOLECULAR-WEIGHT FORM OF DNA TOPOISOMERASE-I IN AMPHIBIAN OOCYTES

Xenopus oocytes express a 165 kDa variant of DNA topoisomerase I (topo I) as opposed to the canonical 110 kDa form of somatic cells (Richard and Bogenhagen, Dev. Biol. 146: 4-11, 1991). By immunofluorescence mi croscopy using variant-specific antibodies we show that this high mole cular weight form is associated with lampbrush chromosome loops and th e inner regions of the amplified nucleoli. Inhibition of topo I-activi ty by either Camptothecin-treatment or microinjection of neutralizing antibodies resulted in loop retraction and the condensation of chromos omes and amplified nucleoli. These data indicate that the oocyte-speci fic 165 kDa form of topo I is involved in transcriptional processes me diated by RNA polymerase I and II and is therefore functionally equiva lent to the somatic cell 110 kDa counterpart.