Post-fertilization polyadenylation during transcriptive and translational inhibition

Cell-free cytoplasmic polyadenylation of oogenic RNA

The products of cell-free ATP incorporation mediated by cytoplasmic fractions prepared from unfertilized sea urchin eggs, anucleate egg halves, nucleate egg halves, emetine-treated fertilized eggs, and four-cell embryos have been characterized to determine to what extent the polymers synthesized are poly(A) and to assess the size distribution of the primers adenylated. As judged by alkaline lability, ribonuclease resistance, and retention on poly(U)-impregnated filters, greater than 92% of the label recovered after RNA extraction is present in poly(A). LiCl fractionation indicates that little, if any, free poly(A) is synthesized or cleaved from RNA primers during the reaction, and that 4S RNA is not an effective initiator. In excess of 85% of the poly(A) is associated with RNA having S-values greater than or equal to 18S. Sedimentation profiles of RNA adenylated in the unfertilized egg and anucleate egg half reactions are identical. Suppression of in vivo protein synthesis by emetine alters the profile of RNA subsequently adenylated in vitro. It is proposed that the apparent constraints on the utilization of cytoplasmic RNA or ribonucleoprotein primers of oogenic origin may be effected by RNA-associated proteins capable of regulating the selection and/or extent of their polyadenylation during early embryogenesis.

Postfertilization poly(A) . protein complex formation on sea urchin maternal messenger RNA

A two-fold increase in polyadenylate [poly(A)] content occurs between fertilization and the two-cell stage in sea urchin zygotes. In this report the role of this cytoplasmic polyadenylation process in the provision of binding sites for poly(A)-associated proteins during early development of Lytechinus pictus is evaluated. Protein-associated poly(A) sequences, from ribonuclease-treated, post-mitochondrial supernatants of various developmental stages, were collected by nitrocellulose filtration and quantified by 3H-poly(U) complex formation. The proportion of protein-associated poly(A) rose from about 27% to about 60% of the total poly(A), on a nucleotide basis, during the period between fertilization and the eight-cell stage. However, the actual increase in number of poly(A) sequences associated with protein was more extensive, about 2.5-fold, since protein-associated poly(A) sequences average about 45 nucleotides longer than free poly(A). The protein-associated poly(A) of eggs and zygotes is found in two types of protease-sensitive complexes which sediment at 8--12 S and 15--20 S. The 8--12 S complex appears to be selectively increased in amount following fertilization. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the poly(A) protein complex fraction indicates the presence of 87,000 and 130,000 molecular weight polypeptides in both eggs and zygotes. It is concluded that quantitative, but not qualitative, alterations in the proportion of protein-associated poly(A) accompanies post-fertilization cytoplasmic polyadenylation in sea urchin zygotes. The attachment of specific proteins to the 3' terminus of maternal RNA's may be involved in their subsequent activities during early embryogenesis.

Synthesis of myosin light chains and accumulation of translatable mRNA coding for light chain-like polypeptides in differentiating muscle cultures

Rat skeletal muscle myosin contains small subunits (light chains, here designated LC1, LC2, LC3) of molecular weight 23,000, 17,000 and 15,000, respectively. The synthesis of myosin light chains during differentiation and the accumulation of mRNA which codes for these proteins were investigated in differentiating rat skeletal muscle cultures. When cultures were labeled prior to cell fusion, radioactive light chains which co-migrated with skeletal muscle myosin light chains on gel electrophoresis were absent or only barely detectable. The low molecular weight peptides which were associated with the heavy chain of myosin extracted from pre-fusion cultures differed in their electrophoretic mobility from light chains of skeletal muscle myosin. Following cell fusion, the amount of labeled LC1 and LC2 increased rapidly. The synthesis of LC3 was barely detectable during cell fusion and never exceeded one-fifth of the amounts of LC1 and LC2 synthesized.

Polyadenylated RNA extracted at different times during differentiation was translated in the wheat germ cell-free system. The products were analyzed on isoelectric focusing-SDS two-dimensional gel electrophoresis, and the radioactivity of the polypeptides co-migrating with myosin light chains was measured. Small amounts of radioactive products co-migrating with LC1 and LC2 became detectable among products of RNA preparations extracted several hours prior to cell fusion. However, the cell-free system directed by post-fusion RNA synthesized much larger amounts of LC1- and LC2-like polypeptides. Rapid accumulation of translatable mRNA for LC1 and LC2 was closely correlated with cell fusion. Radioactive polypeptides co-migrating with LC3 were synthesized in significant amounts in a cell-free system directed by pre-fusion RNA and increased only moderately when RNA extracted after fusion was used.

The dynamics of maternal poly(A)-containing mRNA in fertilized sea urchin eggs

Cytoplasmic polyadenylated RNA with the characteristics of sequestered mRNA exists in the unfertilized sea urchin egg. Following egg activation, the amount of poly(A) doubles, but total RNA content stays constant. Chromatography of the RNA on poly(U)-Sepharose shows that the amount of RNA that bears a poly(A) tract increases slightly (approximately 20-30%) during the 2 hr after fertilization. When a cDNA transcript of the poly(A)+ mRNA from 2 hr zygotes is reacted against poly(A)+ RNA from either eggs or zygotes, the kinetics of reassociation of the two preparations seem identical; hence the RNA sequences bearing poly(A) are the same in eggs and zygotes. Measurement of the length of the poly(A) tract in eggs and zygotes shows an increase in number average length from about 45 bases to 60 bases. Measurement of tract length of poly(A) in two cell zygotes by adenosine/AMP ratios of radioactive RNA shows that the poly(A) tract of the zygote is solely accounted for radioactive RNA, indicating extensive turnover of the poly(A). It is concluded that the poly(A) tract in these cells is subject to both lengthening and shortening, with the former predominating in this instance. the increase in poly(A) does not involve polyadenylation of different sequences, but is due to an increase in the number of polyadenylated sequences and the length of the poly(A) tracts that they bear.