Metabolism of Poly (A) Sequences of Maternal mRNA during Progesterone-Induced Maturation of Xenopus laevis Oocytes

Abstract

Changes in the content and the size of poly(A) sequences during progesterone-induced maturation of Xenopus laevis oocytes were determined by [3H]poly(U) hybridization and by labeling with exogenously supplied radioactive adenosine. The poly(A) content increased significantly (by about 20 %) by the time of germinal vesicle breakdown (GVBD), then decreased continuously to one half of the initial content by the end of maturation. Gel electrophoresis revealed that poly(A) sequences were being elongated by 20-30 nucleotides before GVBD except for the smallest class of poly-(A) (ca. 20 nucleotides long), but after GVBD they were extensively degraded which gave rise to the egg poly(A) sequences of quite different sizes. All these changes in poly(A) content and size also occurred in oocytes which either were treated with actinomycin D or enucleated before exposure to progesterone; this signifies that they were not dependent on the nucleus. Labeling experi-ments on maturing oocytes showed that poly(A) sequences were synthesized actively independent of the concomitant synthesis of RNA even when a substantial amount of the poly(A) sequences was being degraded (after GVBD). The size distribution of these labeled poly(A) sequences was the same as that of steady-state poly(A) measured by [3H]poly(U) binding, and they were associated with non-radioactive heterogeneous high molecular weight RNA. Thus, the present findings indicate that the poly(A) sequences associated with cytoplasmic maternal mRNA were being turned over extensively and that a portion of them was completely degraded during oocyte maturation. This poly(A) turnover also was indicated in full-grown oocytes which were not treated with progesterone and which showed no changes in the content and size of poly(A) throughout the experimental period.