Tissue-restricted accumulation of a ribosomal protein mRNA is not coordinated with rRNA transcription and precedes growth of the sea urchin pluteus larva.

We have identified an mRNA that encodes a protein, SpS24, of the small ribosomal subunit in the sea urchin, Strongylocentrotus purpuratus. RNA blot and in situ hybridization analyses show that the SpS24 gene is active during early oogenesis, downregulated in the mature egg and during cleavage, and reactivated in the early blastula. The mRNA then increases in abundance at least 100-fold. Later in development, expression of SpS24 mRNA becomes restricted primarily to cells in the oral ectoderm and endoderm of the pluteus larva, and the message is undetectable in aboral ectoderm cells and most mesenchyme cells. To determine whether transcription of the ribosomal RNA genes occurs at a higher rate in oral ectoderm and endoderm tissues, a probe for the transcribed spacer was used in RNase protection and in situ hybridization assays. High concentrations of rRNA-processing intermediates were observed in unfertilized eggs and shown to reside primarily, if not exclusively, in the cytoplasm. The spatial and temporal distributions of these sequences strongly suggest that they are associated with heavy bodies. New embryonic rRNA transcripts are first detectable at the very early blastula stage. In later embryos, the content of this transcribed spacer sequence is similar in all but a few cells, which implies that they synthesize rRNA at a similar low rate. Comparison of available estimates of rRNA transcription rate with the potential rate of SpS24 protein synthesis, calculated from SpS24 mRNA prevalence, shows that oral ectoderm and endoderm cells have the capacity to synthesize 15- to 30-fold more SpS24 protein than is required to keep pace with rRNA synthesis in these cells. Because the sea urchin embryo develops from an egg to a pluteus larva in the absence of growth, this stockpiling of SpS24 mRNA anticipates rather than accompanies the onset of growth, which does not begin until after feeding. Upregulation of this gene is therefore part of the developmental program, rather than a physiological response to nutrient availability.