Differential accumulation of two size classes of poly(A) associated with messenger RNA during oogenesis in Xenopus laevis

The RNA of full-grown oocytes of Xenopus laevis contains two distinct size classes of poly(A), designated poly(A) S and poly(A) L, which contain 15–30 (mean = 20) and 40–80 (mean = 61) A residues, respectively. Both poly(A) L and poly(A) S are associated with RNA which is heterogeneous in size. The two classes of poly(A) + RNA can be separated by affinity chromatography: Only poly(A) L + RNA binds to oligo(dT)-cellulose under appropriate conditions, but up to 50% of the poly(A) S + RNA can be isolated from the void fraction by binding to poly(U)-Sepharose. Both classes of poly(A) + RNA are active as messenger RNA in an in vitro system and yield identical patterns of in vitro protein products. Previtellogenic oocytes contain almost exclusively poly(A) L, which accumulates up to vitellogenesis but remains almost constant in amount (molecules/oocyte) during vitellogenesis and in the full-grown oocyte. Poly(A) S accumulates (molecules/oocyte) from early vitellogenesis up to the full-grown oocyte. The total number of poly(A) + RNA molecules per oocyte increases throughout oogenesis from 2 × 10 10/previtellogenic oocyte [80–90% poly(A) L] to 20 × 10 10/full-grown oocyte (25–40% poly(A) L). It is argued that poly(A) S is protected from degradation in the oocyte, thus stabilizing the “maternal” poly(A) + mRNA.