Synthesis of ribosomal RNA during sea urchin development. II. Electrophoretic analysis of nuclear and cytoplasmic RNA's

Studies on the structure and possible function of the RNA "cap" in developing sea urchins

Paracentrotus lividus embryos at the hatching blastula stage very quickly incorporate radioactivity from labeled nucleosides (except uridine) or 32P- or methyl-labeled methionine into a portion of the RNA that has been identified as a "cap". The most probable sequence of this cap is m7G (5') ppp (5')mAmpCp. A very active "capping" and methylation of the "cap" of preexisting RNA molecules was shown to occur at the blastula stage.

The RNA of unfertilized sea urchin eggs is "capped"

It is shown that the RNA of unfertilized sea urchin eggs is active in stimulating protein synthesis in a wheat germ cell free system. This activity is not lowered by conditions that inhibit the methylation processes but is inhibited by a treatment that damages the "cap", A difference in activity in a wheat germ cell free system between the RNA of unfertilized eggs and the RNA from early embryos is described.

Methylated constituents of poly(A)- and poly(A)+ polyribosomal RNA of sea urchin embryos

We have examined the poly(A)- and poly(A)+ polyribosomal (Ps) RNAs of developing sea urchin embryos to see whether they contain the capped structures previously observed in the poly(A)+ cytoplasmic RNA (mRNA) of a variety of eucaryotic organisms. We have found that both classes of Ps RNA contain 7-methylguanosine linked to the RNA by a 5'-5' triphosphate bridge. Capped structures derived from either class of Ps RNA appear to be of the type m7G5'ppp5'NmpNp, with only a single sugar-methylated nucleoside per cap, and we have estimated that both classes of Ps RNA are capped to the same extent. The 5' terminal 2'-0-methylated nucleosides have been identified and quantitated. The relative amounts are Am = 78%, Gm = 14%, Cm = 5%, Um = 3% for poly(A)- Ps RNA, and Am = 78%, Gm = 15%, Cm = 5%, Um = 2% for poly (A)+ Ps RNA. In addition, both classes of Ps RNA contain 2-4 internal residues of N6-methyladenylic acid per molecule.

Stabilities of nuclear and messenger RNA molecules in sea urchin embryos

The kinetics of accumulation of radioactive adenosine in adenosine triphosphate and in RNA of nuclear, cytoplasmic, and polysomal fractions of sea urchin embryos have been analyzed. 85% of the RNA synthesized decays in the nucleus with an apparently uniform half-life of about 7 min. The remaining 15% goes to the cytoplasm, mostly entering polysomes, and decays with a quite uniform half-life of about 75 min. The nuclear RNA accounts for one-third and the cytoplasmic RNA accounts for two-thirds of the total unstable RNA which accumulates at steady state in the embryo. The size distribution of short-labeled nuclear RNA is very similar to that of long-labeled messenger RNA, when both are extracted directly from the cells without a previous cell fractionation.

The effect of protein synthesis inhibition on the entry of messenger RNA into the cytoplasm of sea urchin embryos

Emetine is a potent inhibitor of protein synthesis in sea urchin embryos. At a concentration of the drug that rapidly inhibits protein synthesis in blastulae by 95%, uridine incorporation into RNA continues for more than 1 hr and presumptive histone messenger RNA is synthesized and transported into the cytoplasm where it is apparently associated with polyribosomes. Possible explanations of this result and its implications for the "informasome" theory of messenger transport in embryonic cells are discussed.

Synthesis of RNA in isolated nuclei of sea urchin embryos

It is demonstrated that isolated nuclei of sea urchin embryos are able to incorporate radioactive nucleotides into RNA.Some properties of the incorporation system are described.