RNA synthesis during the development of Xenopus laevis, the South African clawed toad

Abstract

Methods have been described for radioactive-labeling, purification and characterization of high molecular weight RNA from eggs and embryos of Xenopus laevis , the South African clawed toad. The different functional classes of RNA have been distinguished by their sedimentation coefficients, rates of synthesis and accumulation in the embryos, as well as by their nucleotide composition. When RNA is isolated at pH 5 under conditions which minimize its degradation, DNA-like RNA is heterogeneous, having sedimentation coefficients of 18 s and larger. However, DNA-like RNA can be resolved in a partly degraded form from high molecular weight ribosomal RNA precursor molecules as well as 28 s and 18 s ribosomal RNA by its isolation in association with the ribosome particles. Deoxycholate is required to effect this isolation. With the use of these techniques, the following sequence of developmental events has been described. The mature X. laevis oocyte synthesizes a small amount of heterogeneous RNA as a response to hormone injection during the period of ovulation. Some of this RNA can be isolated in association with the ribosomes of the unfertilized egg. Fertilization does not alter this pattern. During the cleavage stages, DNA-like RNA, soluble RNA and DNA are synthesized to the exclusion of ribosomal RNA. Synthesis of new ribosomal RNA begins with the onset of gastrulation at the same stage that the nucleolus first appears. As development proceeds, ribosomal RNA synthesis gradually increases, but the total content of RNA in the embryo does not increase significantly until after hatching. By the completion of embryogenesis the total RNA in the embryo is about double that present in the unfertilized egg.