RNA transcription and translation in sea urchin oocytes and eggs

Sea urchin oocytes and mature unfertilized eggs synthesize 4 S RNA, histone mRNAs, mitochondrial RNAs, and heterodisperse RNAs. Incorporation into cytoplasmic ribosomal RNAs was not detected. Oocytes, but not mature eggs, synthesize heterogeneous, high-molecular-weight RNA. In both oocytes and eggs, a fraction of the newly synthesized histone mRNA is found on polyribosomes. About 50% of the histone mRNA which is pulse-labeled in oocytes is engaged with ribosomes, while the mature egg recruits only about 10% of the newly synthesized histone mRNA onto polysomes. Translation of histone mRNA in eggs was confirmed directly by showing that unfertilized eggs synthesize histones in vivo. Quantitatively, mitochondrial transcription contributes most prominently to the pattern of total RNA synthesis, accounting for approximately 70-90% of the [³H]uridine incorporation by oocytes and eggs. Labeled mitochondrial RNAs include at least 10 discrete poly(A)⁺ RNA species, presumably mRNAs, and 2 poly(A)^- ribosomal RNAs. It is extremely difficult to completely eliminate all traces of labeled mitochondrial RNA from 15,000g "postmitochondrial" supernatants in oocytes, eggs, and very early embryos: neither repeated pelleting at 15,000g nor attempts to selectively inhibit mitochondrial transcription using ethidium bromide and acridine orange (which also depress nuclear transcription) were successful. These labeled mitochondrial RNAs, which routinely contaminate cytoplasmic 15,000g supernatants, could pose problems for workers interested in analyzing the synthesis and utilization of abundant poly(A)⁺ and poly(A)^- RNAs in oocytes, eggs, and early embryos.