Multiple Forms of Deoxyribonucleic Acid-dependent Ribonucleic Acid Polymerase in Xenopus laevis

The amounts of the various forms of RNA polymerase (Ia, Ib, IIa, IIb, and III) have been determined in oocytes, eggs, and embryos of Xenopus laevis. During oogenesis the relative proportions of the enzymes remain nearly constant, although the absolute levels of all forms increase dramatically. As a result, the mature oocyte, as well as the unfertilized egg, contains 4 to 5 orders of magnitude more RNA polymerase activity than is present in an individual somatic cell. Both oocytes and eggs contain unusually high levels of forms III and IIa in addition to forms I and IIb which predominate in somatic cells. The amounts and relative proportions of the five enzymes in the unfertilized egg remain about the same up to gastrulation of the embryo. During subsequent stages of embryogenesis the total amounts of forms I and IIb increase about 10-fold while the amounts of forms IIa and III increase only slightly. The rapid increase in cell number along with this selective increase in forms I and IIb during early development re-establishes the same relative and absolute enzyme levels per cell in the swimming embryo that are present in adult somatic cells. Anucleolate mutant embryos at the early swimming stage have the same amount of each enzyme as do control embryos. Furthermore the relative amounts of the various enzymes in purified nuclei are the same in anucleolate embryos as they are in control embryos. Chromatographic heterogeneity in enzyme I was found at all stages of oocyte maturation and throughout embryogenesis of control and anucleolate mutant embryos.