Triphosphate residues at the 5' ends of rRNA precursor and 5S RNA from Dictyostelium discoideum.

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Abstract
Direct evidence was presented for the preservation of a transcriptional initiation sequence in a eukaryotic rRNA precursor: the 5''-end group for precursor to 17S rRNA (p17S RNA) from D. discoideum is identified as the triphosphate residue pppA-. Mature 5S RNA from Dictyostelium bears a different triphosphate residue, pppG-. In contrast, there was no evidence for more than 1 phosphate at the 5'' end of the 25S rRNA precursor (p25S RNA). Apparently synthesis of the large ribosomal RNAs of Dictyostelium begins with the 5''-terminal sequence of the p17S RNA, and 5S RNA transcription must be initiated independently, despite the close association of the 5S and rRNA coding segments.