We present here direct evidence for the preservation of a transcriptional initiation sequence in a eukaryotic rRNA precursor: the 5'-end group for precursor to 17S rRNA (pl7S RNA) from Dictyostelium discoideum is identified as the triphosphate residue pppA-. We also show that mature 5S RNA from Dictyostelium bears a different triphosphate residue, pppG-. In contrast, we find no evidence for more than one phosphate at the 5' end of the 25S rRNA precursor (p25S RNA). These observations indicate that synthesis of the large ribosomal RNAs of Dictyostelium begins with the 5'-terminal sequence of the p17S RNA, and that 5S RNA transcription must be initiated independently, despite the close association of the 5S and rRNA coding segments. Although the sequence of events in rRNA production is similar in all organisms, certain characteristic variations in the pathway distinguish eukaryotes from prokaryotes. In bacteria, the genes encoding the 23S, 16S, and 5S RNAs are transcribed consecutively from a single promoter in the order 16S-23S-5S (1-3).
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