RIBOSOMAL RIBONUCLEIC-ACID SYNTHESIS AND PROCESSING IN EMBRYOS OF THE HAWAIIAN SEA-URCHIN TRIPNEUSTES-GRATILLA

Pulse-labeled RNA from plutei of the sea urchin Tripneustes gratilla was fractionated in sucrose gradients and sodium dodecyl sulfate-polyacrylamide gels and hybridized to recombinant plasmid DNA containing a portion of the T. gratilla ribosomal gene to identify a large, ribosomal RNA (rRNA) precursor of 2.5 x 106 daltons. Both pulse-chase and incorporation experiments indicate that the 2.5 x 106 dalton moiety is sequentially processed to 1.6 x 106 and 1.0 x 106 dalton intermediates and then to mature 1.45 x 106 (26 S) and 0.7 x 106 (18 S) dalton species. As judged by the respective molecular weights of the precursor and mature rRNA species, processing is nonconservative with approximately 15% of the precursor mass degraded and 85% contributing mature 26S and 18S rRNA. The absolute rate of synthesis of the rRNA precursor was measured by quantitating the flow of radioactivity through the embryos' GTP pool and into each of the rRNA species identified above. The precursor is synthesized at a rate of 3100 molecules h-1 nucleus-1 and is present at a steady-state concentration of 1450 molecules/ nucleus. Its half-life is 19 min. Comparison of the rate of synthesis of the precursor with the rate of accumulation of mature 26S and 18S rRNA shows that most precursor molecules yield mature rRNA. The number of copies of the ribosomal genes in the T. gratilla genome was measured by comparing the rates of reassociation of T. gratilla ribosomal DNA (rDNA) and single copy DNA with total T. gratilla DNA. These results indicate that the ribosomal genes are repeated 50 times per haploid genome. The ribosomal genes in pluteus cells are thus transcribed at an average rate of about 31 copies sequence-1 h-1. © 1979, American Chemical Society. All rights reserved.