Nuclear RNA synthesis in sea urchin embryos

In vivo protein phosphorylation and labeling of ATP in sea urchin eggs loaded with 32PO4 via electroporation

Protein phosphorylation was examined in sea urchin eggs in which the ATP was labeled with 32P over a brief period of time using reversible electrical poration to gain access to the cytoplasm. Unfertilized eggs from two species, Lytechinus pictus and Strongylocentrotus purpuratus, were electrically permeabilized and incubated in the presence of [32P]H3PO4, under conditions allowing label uptake. After a 5-min loading period the eggs were resealed and the fate of the label was monitored. The label had equilibrated with the cellular ATP pool within the 13-min period required for loading and resealing the eggs. Furthermore, this equilibrium was maintained for at least 2 hr beyond the loading period in either unfertilized or fertilized eggs (i.e., the specific activity of ATP was the same for fertilized and unfertilized eggs). We also examined the position of the label within the ATP and found that 40-45% of the label isolated with the ATP was in the gamma phosphate of ATP and hence was immediately available for protein phosphorylation. The label was maintained in this position in the ATP for at least 2 hr following the loading period and was not affected by fertilization (determined for L. pictus only). The phosphoprotein banding pattern was determined by gel electrophoresis and autoradiography at various time points following the loading period. There was a continuous increase of label incorporated into protein over time; however, the banding pattern did not change. This pattern was not affected by fertilization. Furthermore, inhibition of protein synthesis (with emetine) had no effect on this phosphoprotein banding pattern. Although the loading period was brief there was sufficient incorporation of label into protein during this time to obscure potential regulatory phosphorylation events.

Small nuclear RNAs in cellular growth and differentiation. I: metabolic alterations seen in Friend erythroleukemic cells

Electrophoretic analysis of near steady-state labeled nuclear RNA obtained from Friend virus-transformed murine erythroleukemic cells reveals the presence of at least 15 small nuclear RNAs (snRNAs) distinct from ribosomal 5.8S or 5S. Identical qualitative distributions were obtained from logarithmically growing, stationary-phase, and dimethyl sulfoxide-induced, terminally differentiated cultures, indicating the constitutive synthesis of all snRNAs regardless of the proliferative or differentiated state of the cells. However, several quantitative differences in nuclear snRNA levels were observed. Progression from rapidly growing to stationary-phase cultures was accompanied by the marked reduction in accumulation of all snRNAs except the 4.5S snRNAs. Particularly striking were the decreases in levels of U3 and the U1 group, snRNAs that are relatively abundant. Similar reductions were noted when cells were induced to differentiate, except that decreases in the levels of U2 and 4.5S were more dramatic than those seen for cells entering stationary-phase. The data thus demonstrate that snRNA levels may be regulated both in association with changes in proliferative capacity of cells and with changes in gene expression that occur during terminal differentiation.

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.

The programmed switch in lysine-rich histone synthesis at gastrulation

The pattern of histone synthesis changes during development in sea urchins. One change involves two different lysine-rich histones. In Lytechinus, the late appearing histone, H1g, begins to be synthesized at gastrulation. Preformed ("maternal") mRNA of unfertilized eggs contains sequences that direct the synthesis of a part of the histone made during the period of cleavage. During that interval a different lysine-rich hsitone, H1m, is produced. Maternal mRNA is not, however, the sole source of templates for histone synthesis. Transcription of embryonic genomes provides the major translate both maternal and embryonic messages. A cell-free system derived from wheat germ was used to test RNAs from unfertilized eggs and from post-gastrula embryos for their capacity to direct the synthesis of histones, including the H1 types in vitro. Maternal mRNA includes templates for the synthesis of H1m, but it appears to lack those for H1g, since the cell-free system yields only H1m when challenged wigh egg RNA. Since the cell-free system is capable of translating H1g mRNA when presented with it, the synthesis of H1g is probably a very early developmental event controlled at the level of transcription.

Individual histone messenger RNAs: identification by template activity

Newly synthesized polysomal messenger RNAs from cleavage stage embryos of the sea urchin Arbacia punctulata and Lytechinus pictus that contain putative histone mRNAs have been fractionated on 6% polyacrylamide slab gels. At least 8 RNA species with unique electrophoretic mobilities have been recognized. The complex of RNAs has been eluted from the gels in three groups, A, B, and C, in increasing order of mobility. The template activity of the three fractions and the unfractionated starting material was examined in the mouse Krebs II ascites tumor cell-free protein synthesizing system. The unfractionated messenger complex programs the synthesis of proteins that co-electrophorese exclusively with sea urchin histones in both sodium dodecyl sulfate and acid urea gel systems. The products of in vitro protein systhesis stimulated by the individual polyacrylamide gel RNA fractions were similarly examined. Each stimulated protein synthesis and was enriched for specific histone templates. We conclude that RNA fraction A is template for histone f1, C is template for histone f2a1, and B serves as template for f2b, f2a2, and f3 histones. A minor degree of contamination of the A and B RNA fractions was obvious from the production of other histones by each template. The co-electrophoresis of specific template activity with specific radiolabeled RNAs supports the concept that most or all of the labeled RNAs are indeed themselves the histone mRNAs.